This paper deals with a model of the firm engaged in production to order. Models of this situation frequently conclude that the size of the order and production rate each influence the costs of production. Despite this fact, empirical work on production costs focus almost exclusively on either production rate or the size of the order as explainers of variations in costs. In this paper, a model is developed which is consistent with both these facts. The model permits production rate to be a decision variable in the manufacturer's production planning. The model is solved for an optimal production plan which requires production rate to change throughout the life of the program. The planning situation, where alternative order sizes are considered, is then contrasted with the production situation, where unit costs are related to cumulative output given a production plan. This approach provides guidance for the use of data on past programs in the estimation of cost functions and the evaluation of future production plans.