For this achievement you must have 500,000 gold on your persons. I got this while leveling up on the 20th level. Each time I played I'd get anywhere from 1-5 items. The items you pick up usually depend on the items you have on the last level. So you'll either get the exact items you have on, or a better one.
Just sell off the extra items and replay the level until you hit 500,000.
(Side note: For this achievement it didn't unlock when I hit 500,000. I had to leave the merchant and press )
If you hit 500,000 in a level from pick-ups, the achievement wont unlock until you get to the stat screen.