To determine the cost of expanding blood product operations, the concept of marginal cost must be used. This article reports the development and implementation of a method of costing increases in collecting plasma using apheresis operations. The model takes into account the fact that certain resource inputs--notably, direct labor and machines--increase in discrete steps rather than in a continuous manner. To address this fact, a stepwise cost analysis function was developed, which related operating costs to the volume of apheresis collections. This function was used to predict the marginal costs of potential increases in the supply of plasma at a blood center in Canada. Differences were noted in the cost of plasma in Canada and the United States, much of which could be attributed to different standards regarding the volume of plasma per collection and to differences in pricing materials.