i bet if it was well enough designed it could be limited by things like:
only counting the team that wins
using the % chance of a units use as its % influence over costs (so experimenting with units has less effect then using them consistently)
a means to stabilize the balance:
maybe waves ~ balance influence occurs @ 5% per day.. so that a 20 day cycle is one meta cycle inline with new units and stuff
a way to mitigate inflation =P
to be honest idk how all the issues you mentioned get solved but if they did get solved after a while the game would present you no units not considered effective for the price
but maybe that's not fun > kinda like pokemon has useless status moves for stupid people to instantly identify.. if pokemon moves are all super balanced then players would sweat and develop anxiety
too much strategy makes the right choices less clear cut and this makes it more like 'GO' (the chess like game) and less like beer pong
fads would strangely get balanced too.. so if it was fun to make reef every game they would cost more until it was average fun =/