The economy of scale for multiunit inertial fusion energy (IFE) power plants is explored based on the molten salt HYLIFE-II fusion chamber concept, for the purpose of producing lower cost electricity and hydrogen fuel. The cost of electricity (CoE) is minimized with a new IFE systems code IFEFUEL5 for a matrix of plant cases with one to eight fusion chambers of 250 to 2000-MW(electric) net output each, sharing a common heavy-ion driver and target factory. Improvements to previous HYLIFE-II models include a recirculating induction linac driver optimized as a function of driver energy and rep-rate (average driver power), inclusion of beam switchyard costs, a fusion chamber cost scaling dependence on both thermal power and fusion yield, and a more accurate bypass pump power scaling with chamber rep-rate. A CoE less than 3 ¢/kW(electric)·h is found for plant outputs greater than 2 GW(electric), allowing hydrogen fuel production by water electrolysis to provide lower fuel cost per mile for higher efficiency hydrogen engines compared with gasoline engines. These multiunit, multi-GW(electric) IFE plants allow staged utility plant deployment, lower optimum chamber reprates, less sensitivity to driver and target fabrication costs, and a CoE possibly lower than future fission, fossil, and solar competitors.