Sure the moment USF has to pay for Lieutenant, Captain & Major since these units come also for free.
Or UKF can pay for their crappy base howitzers as those are also for free.
I think different, we should rather give Ostheer & Soviets some free stuff when they upgrade.
Ostheer could use a 5th men on grens. Soviets could get some global weapon improvement that increases their dps.
Or we could make USF pay side costs in menpower and fuel for stuff like healing, weapon upgrades or nades, say 500 menpower and 60 fuel additionally in total!
But on the second thought, giving something to vanila armies is also an option, after all it is they who have highest costs and no free perks for it(except for ost upgrades/nades).