@PhoenixofForce Just some random advice for designing and balancing a TD game.
To calculate which is the turret damage output you can use DPS, but if they have different range consider using number of attacks in average times damage per shot. That, divided by cost will rank your turrets from best value to lowest.
Lowest value turrets should be cheaper and used on the start of the game, while best value turrets are those more powerful variants for end-game.
Enemy balancing is also important. For the first few levels I would have made just the quantity of the enemies go up and keep them weak. After I've reached a good amount of them in the screen I would start making them stronger.
For this purpose if you want to make the game have a nice difficulty curve take the expected damage progression formula per level (can be calculated using turret cost and value) and make the enemy progression start later but have more growth to it. ie: turrets grow linearly (y=x) then enemies grow something like (y=(x-10)^1.2).
And that's a nice way to have some theoretical balance in a TD. Then, you'll always need to tweak the values but you'll get very close to decent ones from the start and save a lot of time and headache.