German tanks should cost $6
Here is why tanks can’t be priced at $5 with weak $4 artillery and $4 mechs (hitting at 3 in D12 versus 2 in D6 like in Global 1940):
With $40 Germany can get 8 tanks, with 48 punch on attack, and 48 on defense with 8 hits
With $40 Germany can get 5 art and 5 mechs, with 30 punch on attack, and 40 on defense with 10 hits
Germany would really be crazy to build anything but tanks
Things change a lot if tanks cost $6:
With $48 Germany can get 8 tanks with 48 punch on attack, and 48 on defense with 8 hits
With $48 Germany can get 6 art and 6 mech with 36 punch on attack, and 48 on defense with 12 hits
A smart $48 build might be 4 tanks, 3 art, 3 mech (42 punch attacking, 48 on defense with 10 hits)
Russian mechs should cost $4 and Russian artillery should hit at 4 (inf or mech paired with art still hit at 3)
Russia should be able to do some counterattacking and have enough mobility to move units where they are needed. At $5, Russian mechs are too expensive to be an efficient way to move themselves and artillery units where they are needed (perhaps to attack @3 but more likely to defend @4). Second, Russian artillery attacking @3 is pretty puny given the tank armies they may be counterattacking and considering the historical effectiveness of Russian artillery (i.e. katyusha rockets). Making mechs $4 and increasing their hit power of art to 4 gives you this:
A purely defensive $48 build would be 16 inf @ $3 (32 punch attacking, 64 on defense with 16 hits)
An offensive $48 build without mobility might be 8 inf, 6 art (46 punch attacking, 56 on defense with 14 hits)
A $48 build for mobility might be 6 art and 6 mech (42 punch attacking, 48 on defense with 12 hits)
A very offensive $48 for mobility build might be 4 tanks @ $6, 3 art, 3 mech (45 punch attacking, 48 on defense with 10 hits)
Those tweaks give Germany motivation to build a more realistic force with combined arms, and give Russia some mobility and chance to fight back. If that’s not enough to even things up, maybe try adding fortresses in Stalingrad and Moscow, and minor ICs in Stalingrad and Leningrad.