A very cool game conceptually! I think you really need to make it easier to earn money in general, games like this where you can get as strong as possible thrive on having quick and easy upgrades so that the player can constantly feel short bursts of satisfaction as they keep getting stronger and stronger and stronger. This game should focus less on being challenging and more on offering the player a canvas where they feel as powerful as possible, so I think maybe dropping a coin after every enemy dies would be a good way to achieve this.
On another note, I would highly recommend having a WebGL build for this game to make it a lot more accessible so that many more people can play it! It's not too difficult to make here's a short tutorial video that should tell you everything you need to know in a very clear way!