I figured out that I could get to wave 50 with a single ice tower in the center of the map. Total spent so far: 3360
On my best attempt, I lost at level 75 with 3 max level ice towers and 5 max level nature towers. Total spent @ end: 21,995.
So between waves 51 and 75, I spent 18,635. I can't for the life of me figure out how you'd save up 23k for one of the mega-powerful short-range turrets. I mean, I had a couple-thousand at the end, which means I had just barely grossed enough to have bought/upgraded that single tower.