The cost structure of the CGAM system, a predefined cogeneration system suggested to unify the different methodologies of thermoeconomic analysis, was investigated by using a thermoeconomic method called modified productive structure analysis (MOPSA). An emphasis has been specially put on how the cost structure of the system is affected by the chosen level of aggregation that specifies the subsystems. It has been found that the unit cost of products is dependent on the chosen level of aggregation of the system only when one considers the entropy flow as one of the parameters to determine the unit cost of products. Copyright (C) 2004 John Wiley Sons, Ltd.