These values are embedded into the java code, so they're a bit trickier to edit than those which are exposed in the config files.
Fortunately starsector can compile these on startup if you set the mod files up correctly, so it can be done with a simple text editor and some .java files.
Note that this can increase the initial loading time of the game, but in my experience doesn't have much impact after the game has finished loading, YMMV.
Anyways, I've attached a quick example I made for you that edits the efficiency overhaul mod and makes it super OP at 90% cost reductions.
This made it faster to test and you can edit it by just opening the EfficiencyOverhaul.java file with notepad and editing the variable called "MAINTENANCE_MULT".
Note that there are a few relevant points to editing hullmods this way, but primarily you want to ensure that the hull_mods.csv file's script entry for the hullmod in question points to your modified script.
I can answer questions related to editing hullmods, but making brand new ones is a bit more involved so no guarantees on that front.
Also I don't always respond very quickly, so don't hold your breath.
Edit: Also for editing csv files I recommend something like Ron's Editor:
https://www.ronsplace.eu/products/ronseditorThe free version has worked perfectly with Starsector's csv files for me thus far, and it's a lot easier than trying to edit them in plaintext or Excel or something.
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