A new approach of the combination of exergetic and economic analysis for the complex energy system has been proposed, that is, a general exergy and the corresponding cost balance equation which can be applied to any component of thermal system have been formulated. One of distinct features of this method is that the exergy of a material stream is decomposed into thermal, mechanical and chemical exergy flows and entropy production flow and that a unit cost exergy is assigned to each disaggregated exergy in the streams at any state. This methodology permits to obtain a set of equations w.r.t. the unit cost of various exergies by applying the cost balance equation to each component of the system and junctions. The monetary evaluation of various exergy (thermal, mechanical, etc) costs as well as the production cost of electricity of the thermal system are possible by solving the set of equations. The proposed exergy costing method has been applied to a 1000kW gas turbine cogeneration system. It turns out that the production cost calculated is same as that estimated from the overall cost balance equation, which is concluded as a reasonable result. Also the exergy costing method proposed provides some information on decision about the design and operation of the cogeneration system.