If I invest 1080 mp, 280 fu and 140 munis to kill a single unit while the opponent hasn't done anything to punish this huge resource sink, it should damn well obliterrate the Pak.
The SU-76m barrage used to be weaker, but free. It was a true combination of anti-tank and light artillery.
You'd get a pack of them, they'd be able to counter their weight in tanks no problem, and you couldn't use infantry anti-tank because four SU-76 barrages would fall on your head every time.
They swapped out the SU-76's free barrage with the ZiS gun's more expensive but more powerful barrage to prevent this: it'd cost 140 MU as you said.
Unfortunately, the loss of versatility also killed the SU-76 in 1v1.