i loaded up a random game of mine.
A 9000-ton 1550-bp frigate's missile armament amounted to 2800 tons and cost about 540 bp, including all necessary components for independent operation.
a 127000-ton 1276-bp freighter design from the same game could mount that, upping its tonnage to 130,000 and its cost to 1816 bp. That is a 2.2% armament.
Although the freighter has no armor its huge engine array gives it many times the HTK of the frigate. (Huge engines could do with less HTK, lol.)
You could say go to a 1% armament, a 1300 ton weapons array in this case but then you are looking at pointlessly-small armaments for all but the largest commercial ships. And further complication as you set up Freighter Leader classes that use all their military space for actives.
But a lot of that is besides the point. The problem with maintenance free warships isn't necessarily cost, it's logistics concerns. Maintenance concerns place significant limits on the ability of empires to project power and conduct vital tasks like wormhole blockades. Even the new 7.2 remote maintenance stations require delivery of MSP from an industrial center. In contrast a 'commercial auxiliary' could - paired with a rec center - maintain an indefinite wormhole blockade until the heat death of the universe.
