Cogeneration plants generate more than one product (e.g., electricity and steam) using to some extent common fuel(s) and equipment items. Several approaches have been suggested in the past for assigning the costs associated with these common equipment items and fuels to the products of the plant. Some of these approaches use exergy-based or thermoeconomic methods. The results, however, may vary within a wide range.This paper presents a new exergy-based approach for assigning the fuel(s) used in a cogeneration plant to the individual products of the plant. Combined with a thermoeconomic analysis, this approach provides the costs associated with the product streams. The new approach is more flexible, i.e. it allows engineers to actively participate in the fuel and cost assigning process. As expected, the results obtained with this approach differ from the results obtained from any of the previous approaches, including the exergetic cost theory and all previous thermoeconomic approaches. The application of the new approach is demonstrated using a combined heat and power plant.