A combination of exergetic and economic analysis for complex energy systems is proposed. We derive a general cost-balance equation which can be applied to any component of a thermal system. In this study, the exergy of a material stream is decomposed into thermal, mechanical and chemical exergy flows and an entropy-production flow. A unit exergy cost is assigned to each disaggregated exergy in the streams at any state. This methodology permits us to obtain a set of equations for the unit costs of various exergies by applying the cost-balance equation to each component of the system and to each junction. The monetary evaluations of various exergy (thermal, mechanical, etc.) costs, as well as the production cost of electricity of the thermal system, are obtained by solving the set of equations. The lost costs of each component of the system can also be obtained by this method. The proposed exergy-costing method has been applied to a 1000-kW gas turbine cogeneration system. Our exergy costing method provides information on decisions about the design and operation of the cogeneration system.