The whole dynamic in this game is : Allies pay for fuel sinks but have cheaper tanks , germans pay more for tanks. (Usually having better tanks overall, with weaker infantry)
Not true, all factions pay for their weapon upgrades and grenades through teching. Ostheer does this directly through paying an inflated tech cost (battle phases), which pays for their side costs as well as their tech. Allies pay through side tech, which while overall more expensive than teching the Ostheer way, allows you to make tactical choices and get by with less than 100% investment in your units.
OKW currently pay about the same for tech as USF, but gets their grenades, weapon upgrades, healing, repair, and static defensive building through this cost. This is the reason why the pre-patch OKW has such a dominating force at all times, without sacrificing any lategame armor tech.
Not to mention they get better top tier units out of their T4 than USF, as well as (arguably) better low tier light vehicles.
In conclusion, if you want OKW to get their weapon unlocks and grenades unlocked through teching like Ostheer, they have to pay the prices that come with those upgrades, in the form of more expensive teching costs. Otherwise, the only way to balance them is to give them seperate side tech to pay for these upgrades with.