The aim of this work is to provide a tool for approximation to the actual energy market value. As exergy represents the quality of energy, in the normal procedure for thermoeconomic optimizing of a given thermal system, the exergy of each stream is used as the basic quantity. The usual method evaluates the cost rates appearing in the cost rate balance in terms of the associated rates of exergy transfer and the corresponding unit costs. In this work, the thermomechanical exergy of each stream is divided into two components with very different qualities and hence with different unit costs. The higher quality component can be assigned the same unit cost as work whereas the lower quality component will usually be assigned a lower unit cost. Therefore, the cost rates of a system's input or output streams will be closer to actual economic values. especially in processes with major pressure variations. An example related to combined cycles is included. (c) 2006 Elsevier Ltd. All rights reserved.