Just a quick note: to keep all of your loot, you have to reach any of the exits of the level. You can't simply save and go back to the main menu screen like with skulls and blueprints, or else you will lose all the gold you collected.
The achievement requires you to pretty much buy all of the upgrades in the game. I actually had to buy everything, but different combinations might save you from having to do so. I think I had 120 loot left to spend before buying the last item, so I was just unlucky. If you plan on completing the game 100%, you should be able to get just over 500,000 loot.
As you progress through the levels, the amount of loot you collect increases. So you'll get a lot more gold in the last level compared to the first level.
Hope this helps a bit!