The argument of being doctrinal is a cost is a bit of a stretch. If that were true, than all doctrinal units should be AS cost efficient as the T35/85 and then everyone would be using doctrinal units rather than core units, which was a huge problem in the game a year or 2 ago.
This doesn't make sense, the reason the 85 is more popular than other doctrinal units is because it is a generalist unit that fills a key role and is useful in all situations, rather than a niche unit limited to specific situations. The over-reliance on doctrinal units was largely fixed by making stock options better or more accessible, not by nerfing doctrinal units back to irrelevance.
There are a lot of things that need attention. When I play soviets, I almost always choose a doctrine w/ the 34/85. It's also worth noting that the most widely used soviet Doctrine in the last ESL tournament has a t34/85 in it. I believe it had a large margin of the second most used doctrine.
Wasn't the last ESL tournament prior to the patch, i.e. when the t34/76 was useless? It's hardly surprising that the majority of players will choose a doctrine with a decent medium in it, given that that is a critical capability the faction otherwise just doesn't have. Even after the MG buff that still applies - people pick the t34/85 doctrines because it provides a versatile workhorse medium, something which every other faction has without needing to choose a doctrine.
Anyway I don't see this discussion going anywhere new. After the MG buff it seems reasonable for a very slight cost increase as per the OP's suggestion, but anything more than that would be overkill. Comparisons with the p4 have some use but aren't suitable for drawing strong conclusions because there are just too many differences between the two. If you think the p4 is underperforming then that is a matter for another thread, although on second thoughts perhaps not because that discussion has already been done to death.