If you just want it as a personal thing, go to starsector install, starsector-core/data/hulls, find ship_data.csv and make a backup. THen, on the right side of that, there's the supply cost of all the ships - how much it costs to deploy, how much they cost per month, etc etc. Just make sure you keep the original backed up.
To do it properly, you'd need to set up a basic mod structure (Just look at any other mod's folder), copy that csv file there, and set it to replace in mod_info.json.
But again, if you're just wanting to do it for yourself, it's easier to just edit the core file - just make sure you back up the original.