Since ost rarely goes over T2 and soviets pretty much always have to go for AT nades/molos, the costs equals out.
Well technically yes, if both sides build t2, get all upgrades and healing then the difference is about 50 or 60 manpower.
However.....
Soviets can delay all upgrades ( ex molotovs ) and buildings and push offensively where as Ost will need to tech else they will get caught out. Obviously you can look in opponents base every 30 secs and leave building till last min but that is hard to keep track off.
I am sure you can argue situations to the contary, but most games i play and watch sov outnumber ost early - mid game. If soviets decide to go full spam ( like lenny ) then they can gain numerical advantage ( i am not arguing that there doesn't come a point where it falls down ).
The comments i made earlier in regard to mg related to first 10 mins of game. Also to those that wish to nitpick i should have said cons can outmaneuver mgs if they have numerical advantage due to being mobile as opposed to mgs being largely static in nature.