Okay so pretty much back to editing a weapon cost as stated then...Thanks
You can change the sell value modifier for everything by going to:
BATTLETECH\BattleTech_Data\StreamingAssets\data\simGameConstants
Open the SimGameConstants.json file
Find ShopSellModifier
Change the value to 9 (don't lose you comma though) and you'll be able to get as much cash as you want buying and selling stuff over and over.