Biogas has been used practically as an attractive renewable energy source for various combined power plants as a recent advancement in technologies. Feasibility investigation of a modified organic Rankine cycle coupled with a gas turbine cycle fueled by biogas (60% methane + 40% carbon dioxide) as a heat source is carried out in this study. Thermodynamic and thermoeconomic analysis are employed as the most powerful tools to estimate performance and cost of the system and the results are compared with the basis coupled system. Also, to investigate how the proposed systems perform under any external disturbances, a thorough sensitivity study around the basic operating input parameters was carried out. The results of the second law analysis demonstrated that among all components and for both proposed systems, the combustion chamber had the highest value of exergy destruction rate, followed by the recovery heat exchanger. The proposed combined system could produce net output electricity of 1368 kW, resulting in the thermal efficiency, exergy efficiency and overall product cost of 41.83%, 38.91%, and 17.2 $/GJ, respectively. Moreover, it is found that the thermal and exergy efficiencies of both systems could be maximized with respect to the air compressor pressure ratio and steam turbine inlet pressure, while the overall product cost of the cycles can be minimized with air compressor pressure ratio and gas turbine inlet temperature.