Both ost and sov dont get extras with tech. No free units, forward retreat points, non doc superheavy or base arty pieces.
So i personaly dont see why they should pay the same to be able to go get a heavy.
While I partially agree, your argument is way to simplified, but sometimes also wrong.
UKF base arty behaves like an offmap ability, not like a unit. The USF FRP costs constant POP and thereby MP income (although relatively less), both factions get a complete rooster of units already while teching up, but also lack no important role in the late game (UKF lacks mortar, UKF rocket arty, OKW a cheap and durable AI vehicle as some examples). SOV and OST medics don't cost fuel if you just look at fuel economy.
If we just count the tech structures, SOV and UKF slightly underpay, OKW slightly overpays. But then OST and OKW DO get "free stuff" for teching. Snares, grenades, weapon improvements. All the nice stuff that often is gated behind side techs for Allies (depending on the faction obviously). Not to say that the weapon rack unlock is identical to Grens being able to get LMGs (the racks are better and should cost more), but since you apparently oppose the fact that we should compare full techs, many factions would lack some vital parts.
The other point that Lago already mentioned is that heavies should be balanced against each other just like mediums. Giving some factions earlier access to heavies than others just creates more issues and solves none.