People keep spending additional units of a particular resource on a want until their marginal benefit is their marginal cost.

People keep spending additional units of a particular resource on a want until their marginal benefit is less than their marginal cost.

Explanation:

The marginal principle is based on comparing the marginal benefits and costs of a given activity. The marginal benefit of some activity is the additional benefit resulting from a small increase in activity, such as, for example, the additional revenue generated when a barber shop is open for an additional hour. Similarly, marginal cost is the additional cost resulting from a small increase in activity, such as, for example, the additional costs incurred when keeping a store open for an additional hour.

Based on this, we can say that people continue to spend additional units of a specific resource on a wish until their marginal benefit is less than their marginal cost.