Gold

The main (and only) currency is a small coin emblazoned with a capital "G".

To obtain gold, the player can: kill enemies in game, destroy the chests that spawn rarely, sell items, play defense mode, or buy it using real money.

When collecting gold in game, the color and the size of the coin will tell you how much value it has, as when you pick it up. The rate for Real Money > Gold is 1:2500. Spending more money will land you a small bonus, up to 30% for 100 USD.