This achievement is accumulative. To check it, press
, go to the quest tab, and scroll down the The Rich the Richer. Always check any boxes with green lights for money. Over the course of the game you'll also get some money from completing quests and looting enemies and bosses. You'll acquire a lot of extra weapons, shields and other gear if you pick up everything you find throughout the game. You can sell these at a gun store for lots of money, which also counts towards the Fence achievement. At the later parts of the game, you can get a lot of credits by selling things, as most of your items will be high quality ones. If you play the game normally and do all of this, you should have around 800,000 dollars after you defeat the Destroyer. If you talk to Tannis and complete the quest, she gives you a very large amount of money automatically. This is how I got the achievement.