Economic analysis is necessary to ascertain the practical viability of a pressure-retarded osmosis (PRO) system for power production, but high complexity and the lack of large scale data has limited such work. In this study, a simple yet powerful economic framework is developed to relate the lower bound of levelized cost of electricity (LCOE) to net power density. A set of simplifying assumptions are used to develop an inverse linear relationship between net power density and LCOE. While net power density can be inferred based on experimentally measured power density, LCOE can be used to judge the economic viability of the PRO system. The minimum required net power density for PRO system to achieve an LCOE of $0.074/kWh (the capacity-weighted average LCOE of solar PV in the U.S.) is found to be 56.4 W/m2. Using this framework, we revisit the commonly cited power density of 5 W/m2 to conclude that it is not economically viable because net power density would be even lower. Finally, we demonstrate that fundamental difference exists between power density and net power density, and as a result we recommend using net power density as a performance metric for PRO system.