Why Price Exceeds Marginal Cost

A perfectly competitive firm faces a flat demand curve, so price equals marginal revenue, and it produces until price equals marginal cost. A firm with market power — a monopolist, an oligopolist, a monopolistic competitor — faces a downward-sloping demand curve instead. To sell one more unit it must lower price on all units, not just the last one, so marginal revenue falls below price at every quantity beyond the first. The firm still maximizes profit where MR = MC, but because MR sits below price, the price it charges sits above marginal cost once that quantity is plugged back into the demand curve.

This gap matters because marginal cost measures what the last unit truly costs society to produce, while price measures what a buyer is willing to pay for it. When price exceeds marginal cost, there are buyers willing to pay more than the cost of production who still don't get the good, because the firm stopped short of that quantity to protect revenue on earlier units. The units between the firm's chosen quantity and the competitive quantity never get made, even though their value to buyers exceeds their cost. That unrealized gain is deadweight loss, a triangle sitting between the demand curve and the marginal cost curve.

None of this requires the firm to be greedy or malicious. Any firm facing a downward-sloping demand curve will land here mechanically, simply by maximizing profit. This is why the topic sits at the start of the imperfect competition unit: every later model — monopoly, oligopoly, monopolistic competition — inherits this same price-above-marginal-cost logic, and the size of that gap becomes the standard way to measure how far a market strays from allocative efficiency.