Moonlighter

This achievement is pretty straightforward but note, it is NOT cumulative. You must have 50,000 gold on your person for it to pop. You can tell how much gold you have by looking in the upper left hand corner of your screen.

Simply sell the items you collect from the dungeons in your shop through the merchant portion of the game. You can also convert items you pick up instantly into gold via an item called the Merchant Mirror (found within the Golem Dungeon) but you receive less gold using this method.