Abstract The high cost of organic Rankine cycle (ORC) systems is a key barrier to their implementation in waste-heat recovery (WHR) applications. In particular, the choice of the expansion device has a significant influence on this cost, strongly affecting the economic viability of an installation. In this work, numerical simulations and optimisation strategies are used to compare the performance and profitability of small-scale ORC systems using reciprocating-piston or single/two-stage screw expanders when recovering heat from the exhaust gases of a 185-kW internal combustion engine operating in baseload mode. The study goes beyond previous work by directly comparing these small-scale expanders for a broad range of working fluids, and by exploring the sensitivity of project viability to key parameters such as electricity price and onsite heat demand. For the piston expander, a lumped-mass model and optimisation based on artificial neural networks are used to generate performance maps, while performance and cost correlations from the literature are used for the screw expanders. The thermodynamic analysis shows that two-stage screw expanders typically deliver more power than either single-stage screw or piston expanders due to their higher conversion efficiency at the required pressure ratios. The best fluids for the proposed application are acetone and ethanol, as these provide a compromise between the exergy losses in the condenser and in the evaporator. The maximum net power output is found to be 17.7 kW, from an ORC engine operating with acetone and a two-stage screw expander. On the other hand, the thermoeconomic optimisation shows that reciprocating-piston expanders show a potential for lower specific costs, and since piston-expander technology is not mature, especially at these scales, this finding motivates further consideration of this component. A minimum specific investment cost of 1630 €/kW is observed for an ORC engine with a piston expander, again with acetone as the working fluid. This system, optimised for minimum cost, gives the shortest payback time of 4 years at an avoided electricity cost of 0.13 €/kWh. Finally, financial appraisals show a high sensitivity of the investment profitability to the value of produced electricity and to the heat-demand intensity.