You are confusing cost effectiveness with actual cost.
Its FUEL cost is low. Its mp cost is not.
Its cost effectiveness is still low, despite the low cost. Think of tank version of M-42.
The cost of vehicles is the combined cost of manpower and fuel cost, so one can not only take into account the MP cost and forget the fuel cost. The T-34/76 actually is very cheap compared to other mediums...
----------------MP----FU----Pop---MP ratio---Fu Ratio---Total Cost ratio
OKW PZ4 ----360---150---12-----0.83------0.53----------0.63
Ost PZ4 -----350---125---12-----0.86-------0.64----------0.72
US M4A3 -----340---110---12-----0.88------0.73----------0.79
UKF Crom-----340---110---12-----0.88------0.73----------0.79
Sov T-34/76--300---80----10
Against medium tanks T34/76 is quite cost effective, where it is not cost effective is against heavies...