I suggest to make the amount of units to be deployed variable in a certain range.
Have a minimum and maximum for the amount of units that may be deployed at once and have the exact amount be reduced if the average cost per unit is higher than a specific value and to be raised if average cost of deployed units is low.
This will allow of different tactical approaches: From overwhelming with masses of cheap units to using few high quality troops.
But still having a minimum/maximum of units that may be deployed is essential. It is not about the decision between 20 conscript like units or 2 fully overstrengthed King Tigers but to vary by 3 to 5 (+/-) or the like.