Microsoft Project uses three types of data to compute each assignment's cost value: Average Total Cost, Average Variable Cost, and Marginal Cost.
Average Total Cost: This is the total cost of all resources used for an assignment divided by the number of units or hours of work completed.
Average Variable Cost: This is the cost of only the variable resources used for an assignment divided by the number of units or hours of work completed.
Marginal Cost: This is the additional cost incurred by producing one extra unit or completing one extra hour of work for an assignment.