When the supply curve slopes upward, the buyer must offer a higher price to obtain additional units. If that higher price also applies to units already being purchased, the extra cost of expanding purchases includes both the new unit and the increased payment on earlier units. Consequently, marginal expenditure exceeds the purchase price.
An upward-sloping supply curve makes expansion more costly than simply multiplying the current price by the number of units purchased. Each increase in quantity may require a higher payment, and that increase can apply across existing purchases. This relationship gives the buyer an important reason to limit purchases relative to a more competitive market.
A single buyer can use its substantial demand-side market power to obtain lower prices for a good, service, or productive input than would occur under competition. In labor markets, the corresponding effects may include lower wages and employment. The comparison is therefore useful for evaluating how buyer concentration changes market results.
Labor is a productive input, so a buyer with substantial purchasing power can influence both the wage paid and the amount of labor purchased. A single-buyer structure may produce lower wages or employment than a competitive labor market. This makes monopsony analysis relevant when studying how employer-side market power affects workers and hiring.
Researchers examine whether one purchaser accounts for all or nearly all demand for the relevant good, service, or productive input. They can then consider whether the supply curve slopes upward, since that condition creates the marginal-expenditure effect. This approach helps distinguish buyer power from ordinary purchasing in a competitive market.
Monopsony analysis applies to labor markets, procurement, and other settings in which a purchaser may dominate demand. It helps explain how buyer power can affect prices, wages, or employment. The framework also supports analysis of regulation and policies designed to limit buyer power, especially where market outcomes differ from competitive results.