Cool concept, I like that the money is actually also your bullets, feels like you need to evaluate your options and the risk you're willing to take a little more.
It felt like my bullets were going through the enemies when I shot from too close, so my only strategy was always to just run away, when I actually wanted to get close, especially cause you implemented knock back. Feels like a miss when you can't use a nice mechanic like that.
Adding a dodge of some sort would be nice, feels like sometime an enemy would just spawn next to me and I couldn't avoid it cause it was just there you know. Having the ability to dodge it would make it so the player is rewarded by being skillful and understanding the mechanics.
Also, the music in fight felt a little repetitive, might want to replace to something a little less intense :P
But overall, I liked the aesthetic and the sprites, and the shooting and movement of the character felt good. Good job!