Against everything with less than 200-170 armor, the Jagdpanzer IV is way more cost effective than the Panther due to higher DPM while costing a lot less. Against anything above, the Panther does not have a clear advantage because of its low DPM.
The only thing that makes the Panther arguably more cost effective is that it can also deal decent damage to infantry. Against vehicles however, both units are roughly on par when facing heavy tanks (in terms of costs vs performance), while the Jagdpanzer IV takes the cake against all mediums/premiums/TDs.
If your sole criteria for cost efficiency is DPS in slugging out, the Pak is far more cost efficient. Actually OKW have to 400/135 15 pop for something pak can do for a fraction of the cost.
By the same criteria SU-85 is more cost efficient than M36, which I doubt you will find allot of people to agree.
As a counter to mediums, best thing one can do if his opponent has build a JP4 to counter his medium, is keep it medium in base. The okw player has already payed more for unit that can not contribute anything while paying more upkeep for that unit.
For countering heavier targets all one has to do is build an ATG to keep the JP away.
It will win versus all Allied TDs, and demolishes them with veterancy, two of which are more expensive, so I'd definitely call that cost effective. When fighting Allied mediums, any Allied mediums, the Jagdpanzer IV is the more cost effective choice because the Panther is simply way more expensive without being much better, while the Panzer IV is a liability against any premium tank because of its low penetration.
Another thing to consider is that it has longer range and lower target size compared to the Panther, which should reduce the amount of damage it takes in a fight quite significantly, resulting in much lower repair times.
Versus allied TDs it can keep them away as long as there no atg around but it getting a kill is another story.
Once more the unit is simply not a cost efficient counter to anything and that is why it use primarily in 3vs3 and above.
To sum up:
Vs medium cost more than any of them hardly better than Su-85 or even Stug
Vs heavy armored target inferior to SU-85 not better than Stug
Vs TDs cost about the same with performance similar to an ATG
Imo JP4 is not actually a bad unit it has simply been power creeped by the extremely cost efficient allied TDs that among other things have seen increases in their penetration while the JP4 has not. (and has received several nerfs in vet bonuses instead)
Relic Entertainment
cblanco ★ 
Heartless Jäger 
