This paper presents a mathematical model for the thermoeconomic evaluation of an Organic Rankine Cycle (ORC), which recovers exhaust gases from an internal combustion engine (ICE) based cogeneration plant, currently in operation in Turin, Italy. The model is based on a complete set of experimental data of the plant, for both nominal and partial load operation conditions. The main components of the ORC plant have been modelled and a thermo-economic analysis has been carried out for each component. The aim of the paper is to perform a thermo-economic analysis of the system through the Theory of Exergetic Cost, in order to provide a complete diagnosis of the plant, by quantifying the performances of the whole system and its sub-systems, and by determining the monetary costs and the exergetic costs. The model can represent a useful decision-making tool for the optimal design of similar plants.