In cases where inputs are in high supply at the current market price and the market for inputs is competitive, the marginal cost of an input is roughly equal to the actual cost of acquiring it. So, in such a situation, the principle described earlier can be expressed in terms of comparing the marginal revenue product to price to acquire the input(s). If the number of accountants seeking a job were fairly substantial and competitive, the actual per unit costs involved in hiring one more accountant would be the marginal cost.
If the market of inputs is less competitive, a firm may have to pay a little higher than the prevailing market price to acquire more units because they will need to be hired away from another firm. In this situation, the marginal cost of inputs may be higher than the price to acquire an additional unit because the resulting price increase for the additional unit may carry over to a price increase of all units being purchased.
Suppose the salary required to hire a new accountant will be higher than what the firm is currently paying accountants with the same ability. Once the firm pays a higher salary to get a new accountant, they may need to raise the salaries of the other similar accountants they already hired just to retain them. In this instance, the marginal cost of hiring one more accountant could be substantially more than the cost directly associated with adding the new accountant. As a result of the impact on other salaries and associated costs of the hire, the firm may decide that the highest salary for a new accountant that the firm can justify may be on the order of $50,000, even though the resulting marginal revenue product is substantially greater.
If inputs are available in a ready supply, or there are close substitutes available that are in ready supply, the price of an additional unit of input typically reflects either the opportunity cost related to the value of the next best use of that input or the minimum amount needed to induce a new unit to become available. However, there are some production inputs that may be in such limited supply that even further price increases will not attract new units to become available, at least not quickly. In these cases, the marginal revenue product for an input may still considerably exceed its marginal cost, even after all available inputs are in use. The sellers of these goods and services may be aware of this imbalance and insist on a price increase for the input up to a level that brings marginal cost in balance with marginal revenue product. The difference between the amount the provider of the limited input supply is able to charge and the minimum amount that would have been necessary to induce the provider to sell the unit to the firm is called economic rent.
Suppose a contracting firm was hired to do emergency repairs to a major bridge. Due to the time deadline, the firm will need to hire additional construction workers who are already in the area. Normally, these workers may have been willing to work for $70 per hour. However, sensing the contracting firm is being paid a premium for the repairs, meaning the marginal revenue product of labor is high, and there are a limited number of qualified workers available, the workers can insist on being paid as much as $200 per hour for the work. The difference of $130 would be economic rent caused by the shortage of qualified workers available on short notice.
Economic rent can occur in agriculture when highly productive land is in limited supply or in some labor markets like professional sports or commercial entertainment where there is a limited supply of people who have the skills or name recognition needed to make the activity successful.