Killing enemies and completing waves earns you Xplodium, which after you die and return to the menu can be placed in refineries and converted to coins. Here is where the time element comes into this game. You can refine your Xplodium instantly but earn less coins, or you can put it in specified refineries that take longer (10min, 30min, 24hrs, 72hrs) to get more coin in the end for your wait. If you have the best armor and weapons in the game already, besides power-ups, there is not much to spend cash on. This should come with natural progression. If you tear through a ton of waves without dying, it would be a good idea to deposit that large amount of Xplodium into the 72-hour refinery as you get a 400% coin (more if you have Bro Buffs) return, but you do have to wait three days. Do not worry about this too much, as by the time you get to Level 100, you will have upwards of 1,000,000 coins or more in the bank.