Principles of Microeconomics / Unit 7 MIC 0640 review, 2 to 3 minutes
Why Price Equals Marginal Cost Is the Efficiency Condition
The idea
Price equals marginal cost is the most important equality in this course, and it is worth saying slowly why. Read the two sides. Price, on a demand curve, is the most some buyer would pay for one more unit: the marginal benefit of that unit to the person who values it most. Marginal cost, on a supply curve built from firms' marginal cost curves, is what one more unit costs society in resources drawn from other uses. Where they are equal, the last unit made was worth exactly what it cost, and no unit that would have been worth more than its cost has been left unmade. That is allocative efficiency, and it is the marginal rule of unit one applied to a whole market. Competition delivers it without anyone intending it, because each price-taking firm produces to the point where its own marginal cost meets the price, and the price is what buyers pay. Add the long run and competition delivers productive efficiency too: entry pushes each firm to minimum average total cost, so the quantity is made as cheaply as it can be. The result is a benchmark with a specific meaning. It says no surplus is wasted, given who has the money to express demand. It does not say the distribution of that money is right, and it does not hold when the assumptions fail: when a firm can set its price, when a cost falls on someone outside the trade, when a good cannot be withheld from non-payers, or when one side knows what the other does not. Each of those is a later unit, and each is measured as a departure from this equality.
In practice
Worked example: the strawberry market at twenty-five dollars and seven thousand flats, from a thousand farms. The seven-thousandth flat is bought by someone willing to pay twenty-five, and costs its farm between twenty-four and twenty-eight to grow: benefit and cost match as nearly as whole flats allow. Force output down to five thousand and the last flat is worth thirty to its buyer while costing sixteen; fourteen dollars of value per flat goes unmade across two thousand flats. Force output up to nine thousand and buyers value the last flat at about twenty while it costs its farm above twenty-eight; resources worth more elsewhere are spent on it. Only at price equals marginal cost does neither waste occur.
Where people slip
Three cautions. Price equals marginal cost, not price equals average cost, is the allocative condition; average cost governs whether firms survive, marginal cost whether the quantity is right. The equality is a claim about waste, and a society may prefer a different distribution of income and still want no waste. And the equality fails in exactly the places the outline lists as market failure, which is why unit eight starts by breaking it.
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