In Consumer Surplus, it was explained how most consumers enjoy a surplus of benefits that exceeds the purchase price, which is called consumer surplus, equal to the price that they are willing to pay minus the price that they actually paid. Producers, likewise, also enjoy a surplus.
In a market of sellers, each will have their own cost of production. A producer is willing to produce a product if she can receive a price equal to or greater than the economic cost of producing it. Economic cost not only includes the cost of materials and labor, but also the opportunity cost of the seller's time. Hence, economic cost includes what economists call a normal profit.
Each seller has a different efficiency of producing a product. In a purely competitive market, however, producers are price takers, so they can only participate in the market if their economic cost does not exceed the market equilibrium price. However, because some producers are more efficient than others, they will make more than just the economic cost of their production. They will earn a producer surplus, equal to the actual sale price minus their economic cost of production.
Producer Surplus = Actual Sale Price – Economic Cost
No seller is willing to sell for less than his economic cost, and if a seller's economic cost is equal to the selling price, then he earns no producer surplus, so he is considered a marginal seller, because he would be indifferent as to whether he would produce his product to sell in the market or not. If the market price dropped, he would be the first to leave the market to pursue better opportunities elsewhere.
Because all sellers have some economic costs, no seller will sell for less than this, so the minimum cost of any product will equal the cost of the most efficient producer, so the supply curve begins with the most efficient producers, because they are the ones who can produce for the minimum price. As the market price increases, other sellers who are not as efficient will enter the market as long as the market price is greater than their economic cost of production. Note that the most efficient producers have a maximum producer surplus, while the marginal sellers have no producer surplus.
Economists use the concept of the willingness to sell that is comparable to the consumer's willingness to pay. Obviously sellers will be glad to sell their product for any price higher than their economic cost. Indeed, the higher the price, the greater their willingness to sell, but the market price is limited by what buyers are willing to pay. In fact, the producer surplus is limited by the market price, which is set by competition. So the producer surplus equals the area under the market price above the supply curve.