You can edit the cost of a cheap item, say a Small laser, sell it, then edit the cost back down. Browse to the following folder: YourSteamInstallationFolder\steamapps\common\BATTLETECH\BattleTech_Data\StreamingAssets\data\weapon\

Open the "Weapon_Laser_SmallLaser_0-STOCK.json" file with a text editor, find the: "Cost" : 20000,

Entry and change it to a high number, such as making it ten million, which would get you one million C-Bills per sale back in the shop. Remember to change it back later and always back up any files you edit just in case.