Yes i totally agree with you,i myself use mixed army,few or no ogres atm.
I could suggest the system that MTW2 used,when you have more then 4 units of same type the price for the next keeps rising.
More exactly we could use something like,when you have more then 10 units of same type the next unit will cost double the population.I think this is a good system for unit spam(except dragon,they need static limit).
(ie you have 10 ogres that means 10 pop,ogre number 11 will make your pop 12(10+2),ogre 12 will make your population 16(12+4) etc....if you use the same unit you'll end up with ~20 of them and full 60 pop cap)
"This is no rabble of mindless Orcs; these are Uruk-hai. Their armor is thick, and their shields broad."