Regarding damage vs cost and all that - I had made a spreadsheet, but figured out that anything other than fully upgraded towers aren't worth their while. Upgrade the crap out of one tower before making more - assuming that said tower is able to hit all the targets.
Final upgrade information (Type: Cost/Damage/Range/ROF)
Pellet: 200/400/180/VSlow
Squirt: 450/320/90/Fast
Dart: 375/320/140/VSlow
Swarm: 640/480/75/Fast
Frost: 200/70/75/Slow
M4H's thoughts on each tower
Pellet: Useful for the long-range shots, but be careful with placement. If improperly placed, they can stay locked on to a target they have no chance of destroying in time. Remember that a boss getting through still only costs one life.
Squirt: I never really found this useful - however, a concentrated spiral or back-and-forth weave could benefit from this in tight corridors.
Dart: Splash damage goes up to insane levels at max upgrade - a handful of these lets you quickly dump two or three waves of Creep into the maze, and they'll all get mowed down by a salvo of rockets.
Swarm: A necessary cost to ensure aerial supremacy. The first few levels of fliers can get swatted by Pellet or Squirt, but once you get to the later rounds, the splash damage of Swarm takes over. Don't even think about skimping out on the final upgrade, and make sure there's enough Frost towers around to keep fliers frozen for the entire time they're within range.
Frost: Essential to keep the Fast and Flying waves in check - there should be enough of these to guarantee that the Creep are on ice during the entire trip through your highest-damage area. Whether that be a cluster of Typhoons or a huge circle of death under ICBM guard, always, ALWAYS have Frost towers up.
- M4H