Great game
Loved this game, one of the few games I actually spent time to get to the end (it was quite addiciting).
My only gripe is with some of the weapons. My first choice of weapon upgrade was the spiraller, and it got me through to almost half of the game with that alone. It seemed a bit too powerful, and I was able to farm enough cash to get me the big laser (although I didn't get it). It doesn't do quite as well late game, but it served me EXTREMELY well early game.
The other weapon was the lightning, which was my second weapon. And to my "luck" it was really powerful as well. Once you got it, the game was pretty much a breeze, and I found no other alternative that was as powerful as that one. It's powerful all throughout the game, and yet it's not as hard to get either. It's not as great against bosses, but against mobs, it seemed a bit too good. I think you should make it a lot more expensive, especially since you get cash really easily, and the amount you have left over at the end is tremendous.
Also, the last weapon you can get seems really weak to what its cost is, and maybe you should fix that. I found it underwhelming all throughout the game.
Finally, the weaker weapons that I noticed were the reverse, squarestep and vortex. Reverse seemed underwhelming as well, squarestep was hard to aim and didnt seem to do too much damage, and vortex was hard to control.
Anyway, great game. Maybe you should add more unlockables or lower the amount cash you can get since you can get cash so easily.