Economic studies have generally found that professional sports players have salaries that

should be equal to their marginal revenue product.

Explanation:

This applies to basically all employees that work in competitive markets, their salaries should equal their marginal revenue product.

An employee's salary = the market value of hiring the employee = marginal revenue product

The formula for calculating marginal revenue product = marginal physical product x marginal revenue

where:

marginal physical product = extra units produced by the employee marginal revenue = price of the units produced

For example, a new employee can produce 100 units per day and each unit is sold at $0.75, therefore the employee's marginal revenue product = 100 units x $0.75 per unit = $75 per day