Soviet professors were poor, so it was easy to bribe them to get passing grade. To weed out bribers, some trickery was used by state, so bribers can pay for few years or cheat on tests and then fail an exam anyway. In my class, 36 enrolled, 11 graduated.

Later, people learned that and started to buy diploma: faster, cheaper, no risk of failing the final exam.

my advisor was at Kurchatov and MEPhI in 1989 and I never heard anything about this from him

Engineer from a regional institute received about 100 rubles, worker on a factory about 300 rubles (hehemon class), profesor up to 200 rubles, but profesors from top Moscow univs received 800-2000 rubles of hiden salary.

I heard something about 160 ruble a month student stipend. Although, maybe the parents were supplementing, but he said he had to pay them back for rent.

Maybe it was 80 come to think of it.