Fat: Probably something in ar_stats.dat
Property prices are set by global variables - usually in the $1670 range. If you change the values of these variables then when new properties become available they should have your new prices. Properties that are available at the start of the game have the price already applied during the intro missions. To change these values you would need to modify main.scm and start a new game - or edit the pickup prices with SASE1.00.
Busted/Wasted and Pay'n'Spray: I suspect these are hardcoded into the game engine. I wouldn't be surprised if someone has documented the answer but I doubt you'll learn anything poking around in data files and main.scm.
When I use Save Game Editor, i select a pickup and set the price for example from 10.000$ to 250.000$ it doesn't change. I've tried it on New Game, old saves, doing it before booting up the game, during the game. It doesn't seem to work?
EDIT: Nevermind I forgot to go back to "main tab" and "Save"
Thank you so much, you helped me a bunch!