The design and operation analysis and optimization of complex thermal plants, in terms of both thermal and economical variables, is the subject of Thermoeconomics. Several different thermoeconomical methodologies are presented in the literature, each one based upon different principles and therefore presenting somewhat different results. In this article, four methodologies developed by diverse authors are applied to a simple gas turbine cogeneration system. The results are compared: the importance of the division of exergy into mechanical and thermal components and the allocation of the cost of external irreversibility are discussed.