You mean this http://www.ebay.de/itm/260966019229 ?
Is 782€ ridiculous price for somewhat rare and sought after(?) busboard?
Okay, you get three Mediators for that price.
Don't get me wrong, everyone is free to sell&buy whatever hardware they want at the price they agree, but IMHO a desktop G-Rex4000 is not so rare (A G-Rex 4000T would be, but not a desktop one) and:
-the performance difference with a Mediator in practice is not much noticeable
-If your CS mk3/PPC dies you won't be able to use a different cpu accelerator model.
-Mediator supports PCI USB (even if a Deneb is much better) and G-Rex has a bug that prevents using PCI USB cards (although perhaps an openpci driver could work using dma to gfx card ram but it doesn't work right now)
-More recent software supports Mediator but doesn't support G-Rex (OS4.x supports Mediator and although I prefer MorphOS it's an older version no longer possible to register)
-I already own other PCI busboards so my needs are well catered for.
But really, for 100€? I bet your friend wouldn't sell it for anywhere near that price.
Yeah, it was some years ago, a brand new CSPPC 040/40 was sold for 400€ at ASB (that's the one he bought) and second hand ones could be bought by 350€ (probably because there was no classic OS4 for it). OS4 caused that the price of Classic PPC hardware increased quite a lot. In those times I sold my Macrosystem's Falcon1240 for 120€, Blizzard1230/50 mk4 were sold for as little as 42€ (and nobody wanted them) and all people with A1200/030 were leaving Amiga and switching to peecee.