Market Structures and Price Controls: Monopoly, Ceilings and Floors

How much power a firm has over its price depends on the market it sells in. A farmer selling tomatoes at a busy market has none. A firm that is the only supplier of a service has a great deal.

This topic explains the four market structures and how they differ, then looks at what governments do when they think a market price is unfair: they fix a maximum price (a ceiling) or a minimum price (a floor).

The Institute of Chartered Accountants of Nigeria (ICAN) Business Environment paper tests both parts. You need the features of each structure, and you need to draw and explain price ceilings and floors, including the shortages and surpluses they create.

The four market structures

Perfect competition

Perfect competition rests on four conditions: many buyers and sellers, identical products, free entry and exit, and full information. No single firm can affect the market price, so each takes the price as given. If a firm earns more than normal profit, new firms enter, supply rises and the price falls until only normal profit remains.

The perfectly competitive firm in detail

The syllabus lists four features: a large number of buyers and sellers, a homogeneous product, perfect mobility of resources, which means free entry and exit, and perfect knowledge of all relevant information. Each seller is a price taker. The demand curve facing one firm is horizontal, because it can sell any quantity at the going price, so price, average revenue and marginal revenue are equal. The demand curve for the whole industry slopes downward.

• Short-run equilibrium. The firm maximises profit where marginal cost equals marginal revenue, which here also equals price (MC = MR = P), with marginal cost rising. If price is above average cost, the firm earns supernormal profit.
• Long-run equilibrium. Supernormal profit attracts new firms. Supply rises and price falls until price equals the lowest long-run average cost. Each firm then earns only normal profit, which is zero economic profit.
• The shut-down point. In the short run, a firm that closes still loses its fixed costs. It should keep producing as long as price covers average variable cost, because the surplus over variable cost helps to pay the fixed costs. If price falls below the lowest point of average variable cost, it should close. The marginal cost curve above that point is the firm’s short-run supply curve. In the long run the shut-down point is the lowest point of long-run average cost, and the long-run marginal cost curve above it is the long-run supply curve.

Monopoly

A monopoly is a market with a single seller. It keeps its position through barriers to entry, which stop rivals from joining the market.

• Legal barriers: patents, licences and exclusive rights.
• Economies of scale: one large firm can serve the market at a lower unit cost than several small ones (a natural monopoly).
• Control of a key resource or input.
• Aggressive tactics: predatory pricing and heavy advertising.

Because it controls supply, a monopolist can charge a higher price and sell a smaller quantity than a competitive market would. That can harm consumers. But a monopoly can also bring benefits, such as economies of scale and the profit to fund research.

Controlling monopoly power

• Competition law. In Nigeria the Federal Competition and Consumer Protection Commission enforces the Federal Competition and Consumer Protection Act 2018, which prohibits abuse of a dominant position and anti-competitive agreements.
• Sector regulators. Bodies such as the Nigerian Communications Commission and the Nigerian Electricity Regulatory Commission regulate prices and standards in their industries.
• Price regulation. The government limits what a monopoly may charge.
• Encouraging competition. Removing barriers so that rivals can enter.
• Public ownership. The state runs a natural monopoly in the public interest.

The monopolist in detail

A pure monopoly is one firm producing a good with no close substitutes, protected from new rivals. Its demand curve is the industry demand curve and slopes downward, so to sell more it must cut its price. It has no supply curve of its own, because it chooses price and output together from cost and demand. The syllabus lists the sources of monopoly power as:

• control of the whole supply of a basic input;
• economies of scale, which let one large firm sell below any small rival;
• franchises and patents, which give legal protection to reward research;
• a state monopoly granted to a firm or run by government;
• merger and acquisition, when large firms combine or buy out smaller ones; and
• location, when a market is too small for more than one firm, as with rural banking in many developing economies.

Equilibrium. A monopolist follows the same rule, producing where MC = MR. Because its demand curve slopes down, marginal revenue is below price, so price exceeds marginal cost and the firm earns supernormal profit. Barriers to entry let it keep that profit in the long run.

Compared with perfect competition, price is higher and output is lower, because the monopolist need not operate at the lowest point on its long-run average cost curve.

Using the bakery figures from the topic on costs, revenue and profit: marginal revenue is ₦70,000 and marginal cost ₦50,000 at the fourth batch, and ₦60,000 against ₦70,000 at the fifth. The monopolist stops at four batches and sells them at the demand price of ₦85,000 each, well above the marginal cost of ₦50,000.

Price discrimination

Price discrimination means charging different customers different prices for the same product when the differences are not due to differences in cost. A monopolist can do it profitably only when:

1. price elasticity of demand differs between markets or customer groups, because of differences in income or taste. The firm charges more where demand is less elastic;
2. the cost of separating the groups is lower than the extra profit;
3. customers who pay the low price cannot resell to those asked to pay more; and
4. output is set so that marginal cost equals marginal revenue in each market.

Oligopoly

An oligopoly has a few large firms that dominate the market. Each firm’s decisions affect the others, so firms are interdependent. They tend to avoid price wars, because a price cut that one firm makes is quickly matched, leaving all of them worse off. Instead they compete on advertising, branding, service and new features. Firms sometimes agree to fix prices or share the market. That is a cartel, and it is illegal under competition law.

Monopolistic competition

Many firms compete, but each sells a slightly different product, so each has a small amount of control over its price. Entry is easy, so profit is competed away over time. Firms try to stand out through branding, location and quality.

Monopolistic competition in detail

The syllabus lists four features. Each of the many small firms supplies a tiny share, so none affects its rivals’ choices. Products are differentiated, so each firm has some power over its own price. Entry and exit are free, or nearly so. And because there are many close substitutes, the demand curve for each firm slopes down and is very price elastic, with marginal revenue below it.

• Short run. The firm sets MC = MR and earns supernormal profit if price exceeds average cost. If it cannot cover variable cost at any output, it shuts down.
• Long run. New firms making close substitutes compete the supernormal profit away. The firm ends with price equal to long-run average cost, which is normal profit.

Price controls

Governments sometimes override the market price. They may want to protect consumers from high prices, protect producers or workers from low ones, or raise revenue. The two main tools are the price ceiling and the price floor. Use this schedule for the rice market, where the equilibrium is ₦50,000 a bag and 2,000 bags.

Price ceiling (maximum price)

A price ceiling is a legal maximum price. It has an effect only if it is set below the equilibrium price. A ceiling above the equilibrium price changes nothing.

Suppose the government fixes the price of a bag of rice at ₦40,000. From the schedule, buyers want 2,500 bags but producers supply only 1,500. There is a shortage of 1,000 bags.

Consequences of a ceiling

• Shortages. Demand exceeds supply, so goods run out.
• Queues and rationing. Supplies go to those who queue first or who know the seller, not to those who value the good most.
• A black market. Sellers sell illegally above the ceiling. At a supply of 1,500 bags, buyers are willing to pay up to ₦60,000 a bag, as the schedule shows. So the black market price can exceed the old market price.
• Lower quality and supply. Producers cut quality or leave the market because the price no longer covers their costs.

Real examples include rent control and caps on the price of essential goods. The aim is to help low-income buyers, but the shortage can hurt the very people it was meant to protect.

Price floor (minimum price)

A price floor is a legal minimum price. It has an effect only if it is set above the equilibrium price.

Suppose the government guarantees farmers ₦60,000 a bag. Producers supply 2,500 bags but buyers want only 1,500. There is a surplus of 1,000 bags.

Consequences of a floor

• Surpluses. Unsold stock builds up.
• Cost to the government. If it buys the surplus to support the price, it pays for purchase and storage, and some produce may be wasted.
• Higher prices for consumers, who also buy less.
• Benefits to producers who can sell at the higher price.

A minimum wage is a price floor in the labour market. It raises the pay of workers who keep their jobs. Economists debate whether a high minimum wage reduces the number of jobs, because employers may hire fewer workers at the higher cost.

How a scarce good is shared under a ceiling

When a ceiling creates a shortage, something other than price has to decide who gets the good.

The syllabus lists these methods:

• rationing, for example coupons that must be handed in with the payment
• queuing, whether physical lines for essentials or waiting lists for durables
• a black market, where the good is sold illegally at a price above the equilibrium price
• selling at the seller’s preference, such as first come first served or serving friends and family first
• random allocation, which can defeat the aim of the ceiling because rich and poor alike may be lucky

Minimum wage legislation

A minimum wage is a price floor in the labour market, set above the equilibrium wage so that the lowest-paid workers can afford the essentials of life. At the higher wage, more people want to work than employers will hire, so there is an excess supply of labour.

Some workers lose their jobs, unemployment may rise, and workers and employers may quietly agree pay below the legal minimum to keep jobs. The policy helps those who keep their jobs.

Other forms of intervention

• Indirect taxes raise the cost of a good and reduce demand. Higher taxes on tobacco are an example.
• Subsidies lower producers’ costs and raise supply, as with fertiliser subsidies.
• Quotas limit how much can be produced or imported.

Worked example: a ceiling on rice

1. Check the ceiling bites. The equilibrium price is ₦50,000. The ceiling is lower, so it is effective.
2. Find the quantities. At ₦40,000, quantity demanded is 2,500 bags and quantity supplied is 1,500 bags.
3. Calculate the shortage.
   2,500 − 1,500 = 1,000 bags.
4. Find the black market price. With only 1,500 bags available, buyers will pay up to the price on the demand curve for 1,500 bags, which is ₦60,000.
5. Who gains. Buyers who obtain rice at ₦40,000.
6. Who loses. Buyers who cannot get rice, and producers, who sell 500 fewer bags than before (1,500 against 2,000) at a lower price.
7. Conclude. The ceiling helps some buyers but creates a shortage and a black market, and it discourages production, so supply may fall further.

Market structures and price controls in more detail

The three structures compared

Why monopolistic competitors earn only normal profit in the long run

Suppose a boutique earns supernormal profit. Others see this, and because entry is easy, they open similar boutiques. The demand for each firm’s product falls and becomes more elastic as customers have more substitutes. The process continues until price equals average cost, so supernormal profit disappears. Firms try to keep an edge through branding, quality and service, but these cost money, too.

The price ceiling on rice, step by step

Equilibrium is ₦50,000 with 2,000 bags traded. The government fixes a maximum of ₦40,000. At that price buyers demand 2,500 bags and producers supply 1,500 bags, so there is a shortage of 1,000 bags. Because supply is limited to 1,500, only that many are sold, and the rest of the demand goes unmet. Producers’ revenue falls from ₦100 million (50,000 × 2,000) to ₦60 million (40,000 × 1,500).

Consequences of the ceiling

Better ways to help low-income buyers

Rather than a ceiling, the government can give targeted cash transfers or food vouchers to poor households, so they can pay the market price without distorting supply. It can subsidise inputs such as fertiliser, which lowers costs and increases supply, lowering the price. It can increase supply through storage, better roads and imports in a shortage. Each tackles the problem without causing a shortage.

Controlling monopoly power

Governments can regulate prices through a price cap; enforce competition law that prevents abuse and blocks mergers that create dominance; open the market to new entrants or to imports; tax excess profit; or, for natural monopolies such as water supply, run the monopoly under public ownership or tight regulation.

Exam focus: the price ceiling and the shortage it causes

November 2025 (Question 5) asked for a definition, a diagram and five problems from a persistent shortage. May 2026 and November 2025 Section A also tested the sources of monopoly power.

Definition

A maximum price, or price ceiling, is a limit set by government on the price of a good or service. To protect consumers from very high prices it is set below the equilibrium price. Rent control in cities such as Lagos is an example.

Explaining the diagram

Draw demand and supply with the equilibrium price at OPe and quantity at OQe. Mark the ceiling at OP1, below OPe. At that price, quantity demanded rises from OQe to OQ2 while quantity supplied falls from OQe to OQ1. The gap OQ2 minus OQ1 is the shortage. Because the ceiling is not a market-clearing price, the shortage persists. The examiner described the fall in supply as a leftward shift.

Strictly, it is a fall in quantity supplied along the curve, so describe it in the examiner’s words and show it clearly on the diagram.

Five problems caused by the shortage

• Rationing. Government may have to share the limited quantity, for example with coupons.
• Queuing. People queue for the good, or join long waiting lists for durables such as cars or flats.
• Black or parallel markets. The good is sold illegally at a price above the equilibrium price.
• Selling at sellers’ preferences. Sellers may serve friends and family first, or sell first come, first served.
• Random allocation. Chance decides who gets the good, which defeats the aim of helping both rich and poor.

Sources of monopoly power

Control of the whole supply of a basic input, franchises and patents, and mergers and acquisitions all create monopoly power. Divestiture, which is the sale of part of a business, does not. This was the answer to a Section A question in May 2026.

Exam focus: characteristics, merits and demerits of each market structure

Perfect competition

• Characteristics. A large number of buyers and sellers, so all are price-takers; a homogeneous product; perfect mobility of resources (free entry and exit); and perfect knowledge of all relevant information.
• Demand curve. Each firm’s demand curve is horizontal (perfectly elastic), so price equals average revenue equals marginal revenue. The industry’s demand curve slopes downward.
• Equilibrium. Profit is greatest where MC = MR = P, with MC rising. In the short run the firm can earn supernormal profit. New entrants compete it away, so in the long run the firm earns normal profit with price equal to long-run average cost.
• Shutdown point. In the short run a firm shuts down when price falls below average variable cost. Its short-run supply curve is the marginal cost curve above that point.
• Advantages. Goods sell at the lowest possible price; resources are used efficiently; sales promotion is less expensive; and consumers gain consumer surplus.
• Disadvantages. Firms make little or no profit; inventions and innovation are not encouraged; social costs such as pollution are ignored; and income inequality worsens as larger firms force out smaller ones.

Monopoly

• Characteristics. One firm in the industry, no close substitutes, barriers to entry, and a firm that is a price fixer. It has no supply curve of its own.
• Sources of monopoly power. Control of the whole supply of a basic raw material, economies of scale, franchises and patents, state monopoly, mergers and acquisitions, and location.
• Equilibrium. AR slopes down with MR below it, and equilibrium is where MC = MR, which is below price. Barriers to entry let the monopolist earn supernormal profit in the long run.
• Advantages. No risk of over-production; efficient and full use of resources; lower prices through large-scale production; and incentive for research and innovation.
• Disadvantages. Exploitation of consumers through high prices; restricted consumer choice; and artificial scarcity created by limiting output.

Monopolistic competition

• Characteristics. Many firms, each with a tiny share of output, so no firm affects the others’ price and output decisions; product differentiation, which gives each firm some market power; freedom of entry and exit; and a downward-sloping, highly price-elastic demand curve because there are many close substitutes. MR slopes down and lies below demand.
• Short run. The firm produces where MC = MR and can earn supernormal profit, but shuts down if it cannot cover variable cost.
• Long run. New firms with close substitutes compete the profit away. Price equals long-run average cost, so the firm earns only normal profit.

Monopoly compared with perfect competition

Common mistakes in the exam

• Drawing a ceiling above the equilibrium price, where it has no effect, or a floor below it.
• Mixing up the results. A ceiling causes a shortage. A floor causes a surplus.
• Stating the shortage or surplus without reading both quantities from the diagram or schedule.
• Forgetting the black market as a consequence of a ceiling.
• Describing an oligopoly as having one dominant firm. It has a few large firms.
• Saying that all monopolies are bad. Name the benefits as well as the harms.

Quick revision

• Perfect competition: many sellers, identical goods, free entry, price takers.
• Monopoly: one seller, high barriers, price maker. Controlled by competition law, regulators and price rules.
• Oligopoly: a few large, interdependent firms. Non-price competition. Cartels are illegal.
• Monopolistic competition: many firms, differentiated goods, easy entry.
• Perfect competition: MC = MR = P, zero economic profit in the long run, shut down when price is below average variable cost.
• Monopoly: six sources of power, MC = MR below price, supernormal profit kept by barriers, price discrimination needs different elasticities and no resale.
• Monopolistic competition: differentiated goods, free entry, normal profit in the long run.
• Ceiling below equilibrium: shortage, queues, black market, lower quality.
• Floor above equilibrium: surplus, storage cost, higher prices for buyers, benefit for producers.
• A control works only if it is on the right side of the equilibrium price.

Practice questions

These are original questions written in the style of the ICAN exam. Try each one before you open the answer.

Section A style: multiple choice

Section B style: written questions

A further written question

Model answers

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