Equal cost =/= equal investment though. Both the su76 and stug come from a 2nd highest tier while heavies are tied to the highest AND cost a boat load more.
The StuG is in medium tank tier with the Panzer IV. The IS-2 is in medium tank tier with the T-34/76, the Pershing is in medium tank tier with the Sherman, the OKW Tiger I is in medium tank tier with the OKW Panzer IV.
Heavies are tied to medium tech. Until they get a ~100 FU teching barrier separating them from mediums, they're medium-tier generalists. Trying to balance them as some sort of tier up is like sticking the OKW Panzer IV in Mechanized.
The StuG is in an equivalent tech tier to the Jackson. It's cheaper, but it's also a 50 range casemate with less health and penetration.
It shouldn't do as well against heavily armoured units, but it should
not be almost completely neutered by a
generalist in the same tier.
The SU-76 is a little more difficult. It's not for killing light vehicles: the SU-76 doesn't counter any light vehicle the T-70 doesn't already. It used to be a hybrid light TD/artillery tractor for more defensive play. Then it got a munitions cost on its barrage to stop teamgame SU-76 spam, and since then it's been a dead unit outside of pre-rework heavy stalling.
I think what the SU-76 really needs is for the balance team to decide what it is.
I support increased pen at close range for sure but the units used to invalidate medium armour shouldn't be the same ones to counter heavy armour. That's the whole basis of the Jackson discussions.
The problem is the Jackson is it doesn't have the close-range weakness the other 60-range TDs have. Turning it into some sort of anti-heavy silver bullet is an idiotic design choice. Doctrinal, medium tier heavies should not demand a 140ish FU counter unit that isn't good against anything else. That's a perfect recipe for people rushing heavies every game.
Heavies are currently medium-tier vehicles and until we stop treating them like they're a tier up they're going to play like a medium tank in the light armour phase.