I use the terms differential cost and incremental cost interchangeably. In other words, I believe the terms mean the same thing: the difference in cost between two alternatives. For example, if a company determined that the annual cost of operating at 80,000 machine hours was $4,000,000 while the annual cost of operating at 70,000 machine hours was $3,800,000, then the differential cost or incremental cost of the additional 10,000 machine hours was $200,000.
The term marginal cost refers to the cost of operating for one additional machine hour.
