This is cumulative over the entire game. If you're going for "Full Arsenal" you will easily spend this much money buying all the equipment. To amass this much money, make sure to loot everything along your playthrough, even taking all of the junk. You will also be given money for completing quests, so make sure to do as many side quests as possible. You also will find cash, junk and consumables on bodies after defeating enemies in battle, so be sure to loot their bodies. After unlocking "For The Hoarder", sell all your junk and you should have enough money to spend to reach the $500 mark.