Point is additive value means less when you increase something, but means more when you decrease. Take 100, reduce it per 50% 2 times. Multipicative way make it 25% (amiright?...), addictive - 0. So to prevent situation, where we can literally break the game, can we got lower cap?
Hardcoded, or maybe defined. I mean, isnt everybody agree, that free ship is kinda totally wrong situation?...
What mod combinations are you using, anyway? I don't think the base game let's you get more than maybe 35% reduction in build cost. (10% from governor, 10% from ruler, 10% from supremacy, 5% from tech. Is there a civic that affects it?)
EDIT: devouring swarm, -25%, but their rulers don't get ruler traits. So fanatic purifier (-15%) with a ruler who has military pioneer (-10%) and battleship focus (-20%), the supremacy tradition (-10%), a fleet officer governor (-10%), and that tech (-5%) makes 70%, which I suppose does get within striking distance of 0% with mods like exigency (which has a Starbase building that can reduce costs by up to 35%).
I would be okay with a change where most of the vanilla modifiers were moved over to a new "build efficiency" stat, scaled on the assumption that you normally had a 25% cost reduction, so -5% becomes +7.5%, -10% becomes +15%, -15% becomes +25%, and -20% becomes +35%, so the net effect of the modifier stack from before is +112.5% ships for the normal alloy cost of 1.
Just remember to leave cost reduction in as a variable so that mods that
do intend free stuff can still do that without fucking with ship and module definitions, or whatever compatibility-breaking bullshit they already have to do to make the job AI comprehend their modded traits.