Marginal-Cost Pricing (Socially Optimal Price)
What is Marginal-Cost Pricing (Socially Optimal Price)?
Marginal-cost pricing regulates a monopoly by forcing price down to where demand meets marginal cost (P = MC), the allocatively efficient 'socially optimal' output.
Setting P = MC maximizes total surplus because the last unit's value to buyers equals its cost to produce, eliminating deadweight loss. For a natural monopoly, though, MC lies below average total cost across the relevant range, so charging P = MC means price is below ATC and the firm earns a loss, requiring a government subsidy to stay open. This trade-off (efficiency vs. solvency) is why regulators often retreat to fair-return (average-cost) pricing instead.
Marginal-Cost Pricing (Socially Optimal Price): a worked example
A water utility faces demand P = 50 minus Q, has a constant marginal cost of 10 dollars per unit, and carries a fixed cost of 480 dollars for its pipe network. Marginal cost pricing sets P = MC, so 10 = 50 minus Q gives Q = 40 units at a price of 10 dollars. Revenue is 40 times 10, or 400 dollars. Total cost is 480 plus 10 times 40, or 880 dollars. Average total cost at 40 units is 880 over 40, or 22 dollars, so the firm loses 22 minus 10, or 12 dollars per unit, times 40 units, a loss of 480 dollars, exactly its fixed cost. Left alone the monopolist would set MR = MC, solving 50 minus 2Q = 10 for 20 units priced at 30 dollars. Regulation doubles output and erases the deadweight loss, but only a 480 dollar subsidy keeps the pipes open.
The mistake students make with marginal-cost pricing (socially optimal price)
Two errors show up on the diagram. The first is reading the socially optimal quantity off the marginal revenue curve. Marginal revenue belongs to the unregulated monopolist's own profit calculation, while allocative efficiency sits where demand crosses marginal cost, because demand measures what buyers give up for one more unit. The second is confusing allocative efficiency with productive efficiency and placing the socially optimal price at minimum average total cost. A natural monopoly's average total cost is still falling at the regulated output, so no such minimum is anywhere near the efficient quantity.
Marginal-Cost Pricing (Socially Optimal Price) questions
Why does marginal-cost pricing make a natural monopoly lose money?
A natural monopoly carries huge fixed costs and low marginal costs, so its average total cost declines across the relevant range and stays above marginal cost throughout. Forcing price down to marginal cost therefore drops price below average total cost, and the loss equals the gap between ATC and price multiplied by the quantity sold. Regulators keep such a firm alive with a subsidy, a fixed access charge, or by settling for a fair return price instead.
What is the socially optimal price and quantity for a monopoly?
The socially optimal point sits where the demand curve crosses marginal cost, because the value buyers place on the last unit then equals what that unit costs society to produce. Producing less leaves willing buyers unserved and creates deadweight loss, while producing more means the extra units cost more than anyone will pay for them. Total surplus, the whole area between demand and marginal cost up to that quantity, is as large as it can get.
How can a regulator fund marginal-cost pricing?
Governments cover the shortfall with a lump sum subsidy equal to the firm's loss, financed from general taxation. Two part pricing is the market alternative: charge each customer a fixed connection fee that covers the fixed costs, then price every unit at marginal cost. Both approaches preserve the efficient quantity. The catch with subsidies is that the taxes raised to fund them create distortions elsewhere, and a protected firm has weak incentive to cut its costs.
Formula / Example
This is the live Monopoly sandbox. Drag the curves, or open the full version.
Related terms
Common comparisons
Get AP Econ exam tips in your inbox
Occasional emails with study tips, new interactive graphs, and exam-season reminders. Free, no spam.
No spam. Unsubscribe anytime. Read our privacy policy.
Keep track of what you have studied
A free EconLearn account adds progress tracking, your quiz history, and achievements. Studying here is free either way, and there is nothing to pay for as a student.
Create a free accountAlready have one? Sign in
Last updated