This article presents a model for the analysis of cost efficiency within the framework of data envelopment analysis models. It calculates the cost excess, separating a unit of production from its optimal or frontier levels, and, at the same time, breaks these excesses down into three explanatory factors: (a) technical inefficiency, which depends on the quality of the factors consumed, the type of organization and the factor of human behaviour; (b) the availability of the fixed factors along with their level of utilization and the factors mix; and finally (c) the scale or size of the unit of production. The empirical application is carried out on the departments of the Autonomous University of Barcelona. The results show that departmental costs could be reduced on average by more than 13.46% in the long term.