Pressure-retarded osmosis is a renewable method of power production from salinity gradients which has generated significant academic and commercial interest but, to date, has not been successfully implemented on a large scale. In this work, we investigate lower bound cost scenarios for power generation with PRO to evaluate its economic viability. We build a comprehensive economic model for PRO with assumptions that minimize the cost of power production, thereby conclusively identifying the operating conditions that are not economically viable. With the current state-of-the art PRO membranes, we estimate the minimum levelized cost of electricity for PRO of US$1.2/kWh for seawater and river water pairing, $0.44/kWh for reverse osmosis brine and wastewater, and $0.066/kWh for nearly saturated water (26% wt) and river water, all for a 2 MW production system. Only a pairing of extremely high salinity (greater than 18%) water and freshwater has the potential to compete with wind power currently at $0.074/kWh. We show two methods for reducing this cost via economies of scale and reducing the membrane structural parameter. We find that the latter method reduces the levelized cost of electricity significantly more than increasing the membrane permeability coefficient.