Here's what I mapped out for the upgrades
Fire Tower
$$$ dmg range
18 5 100
20 12 100
30 36 100
50 108 100
80 300 100
600 2056 110
1500 7021 120
Water Tower
$$$ dmg range
10 5 100
20 15 150
40 45 150
80 136 180
160 405 200
320 1215 200
730 3342 200
1000 6684 215
1000 7335 300
Tree Tower
$$$ dmg range
15 3 100
20 15 100
40 45 110
80 136 150
160 405 170
320 1215 190
640 3645 200
1100 9000 215
Flower Tower
$$$ dmg range
3000 9090 150
5000 15050 100
15000 90909 90
That's as far as I know. And the storm tower sucks so I didn't include it. Now for some cumulative costs. If you buy the fire, water, and tree towers all the way up you'll end up with this:
Tower Type - Cumulative Cost - Final Damage - Final Range
Fire - $2298 - 7021 dmg - 120 range
Water - $3360 - 7335 dmg - 300 range
Tree - $2375 - 9000 dmg - 215 range
So basically don't bother with fire towers. Trees are good for their damage, waters are good for their range. The Flower tower is too slow and too limited in range to be of much use, and I can't seem to find a way to get up to the 90909 damage level.