There seems to be a bug. I lost the 34th wave, and had just upgraded my grenades to level 9. After I went back to the upgrade screen, I noticed the upgrade cost was 0, and I upgraded to level 10 for free.
You guys should give us the ability to play earlier waves. I got stuck in the higher levels after beating the game because the enemies got too tough to kill quickly, and it was too tedious to raise enough money to upgrade and make it further. I wish I could go to an earlier stage where the enemies weren't as nasty to raise some cash.
Hey pal! We know and will fix it in the next update.