People keep alluding to it, but lets put some numbers out there:
Assuming that a german player is at tier 2 and wants to get a P4:
Escalate to battle phase 2: 200 mp/35 fuel
Build Tier 3: 160 mp/ 25 fuel
P4: 320 mp/ 115 fuel
total: 680 mp / 175 fuel
Panther:
Escalate to battle phase 2: 200 mp/35 fuel
Escalate to battle phase 3: 200 mp/50 fuel
Build Tier 4: 160 mp / 30 fuel
Panther: 600 mp / 145 fuel
Total: 1160 mp / 260 fuel
T34/85:
no tech
pray that you survive
call in: 360 mp, 130 fuel
So in conclusion: a single p4 costs an additional +320 mp / +45 fuel and a panther is an additional +800mp/+130 fuel. If you wanted to compare 2 T34/85s to 2 P4s, then the P4's would still be an additional +280mp/+30 fuel.
More important than the costs is that this call in lets you go T4, while still having a relatively affordable option of getting T34s to supplement your SU-85s. I havent played enough with this doctrine to make a final call on it yet, but I am liking it so far.
This was most certainly the case for the t34/85 at 7 cup. As I said before the tank was in a unique position in that it did not require teching. Now however I think it will require that as I doubt a sov player can hold out to 9cp without any armor so you almost need to compare the tank coat wise only to the German tanks (p4 or panther) not to the tanks + tech. I don't think the t34/85 should be compared to other callins (kv8 is2 kv1) as it acts more like a standard (mainstay) tank. The cost at 7cp should be high as it prevents the tech need but at 9 it should be lower imo (more in line with p4) as you still pay the tech cost.
I feel like I'm repeating myself a lot :/
But again IMO the call in t34/85 should be gotten rid of and the gun upgrade made researchable to everyone like the Sherman was. Or maybe replace the t34/85 callin with the su100 (I think thats what the soviet mother of all tank destroyer's was called). Have it cost something like 25% more than the su85, be on the same platform, same range, but better DMG and pen. I think that would give good synergy with soviet t3 much like the kv8 trees give good synergy with so t4.