Piercing shots got removed. That's a nerf to the Jagdtiger. A pretty significant one. To compensate that loss, a small rice reduction seems reasonable.
Your idea of making the concrete piercing round free with a longer cooldown seems fine to me as well. That would make any other change irrelevant.
Oh for the 347859173498672165 time!
You NERF unit because its OVERPOWERED.
You do NOT compensate for the NERF, because it would lead yet again to unit being OVERPOWERED for the COST and we'd be BACK AT STEP ONE.
This is why it is called a NERF and not STAT RELOCATION.
And to picture it better to you:
I don't remember Guard Rifles getting any cost reductions or compensations when button and PTRS were nerfed.