Blasted my way through 70-odd waves, quitting when I max'd out my stats and had well in excess of $80k. Surprisingly, the most powerful weapon in the game by sheer DPS is the AK-47-like. It's reliable, reloads quick, has plenty of (pretty cheap) ammo, and does decent damage. The shotgun was the most disappointing weapon for me -- granted, it was also hard to see its pellets.
If the swarm got much thicker than it is now, I'd be forced to move between it and the grenade launcher I think. Speaking of weapons, you should give your flamethrower bullets the same speed as the player and then give them their normal speed. Otherwise, it's not particularly useful to fire ahead of yourself but it makes a great retreating weapon.
Would be useful to know what the powerups do as some of the iconography/effects aren't immediately obvious for some of them (in particular the 'fire directly from your ammo stores rather than your magazine' powerup took me quite a while to figure out). I'd also like to see some sort of stats whenever I'm looking at purchasing them, just to know what benefit I'm going to see out of it. I don't necessarily need to know I'll be going from 3.2 to 4.6 units per second if I upgrade Speed - but I would like to know that each pip is, say, a cumulative 20% speed increase.
Sound design needs a little love, and I'd like a 'reload finished' sound to cap off the 'reload started' sound. Some more polish and this game could be very fun. I'd recommend, for cheap enemy variety, giving each enemy a 20% +/- scale difference when spawned just to make things a little less uniform.
I'd also recommend putting in spawn-fountains, where enemies will continue to spawn indefinitely unless dealt with.