ATI tends to release Mac cards on their own, so if the prices are jacked up like crazy, it's not Apple's fault (in this case... ). Anyway, they released the 3870 Mac & PC Edition for around $200 retail (as far as I know there is no Mac-only version) so I don't see why the 4870 would be treated any differently.
Unfortunately that doesn't work so well if the size of the code you need to flash doesn't fit in the flashROM, which happens. In that case you have to do physical work on the card to install larger flashROM chips, which isn't for the faint of heart. Also, that's not necessarily the only difference--my X800 Mac edition has different ports on the back than any of the PC versions. That would only matter depending on what kind of displays you have, of course.