I have noticed on Amazon UK that the PC disc version is much cheaper than the Mac disc version. As this is a Steamplay game where you buy one version and you can play it on PC and Mac, I don't see any reason why you can't buy a PC version on disc at the cheapest price then use that code in steam to register the game and install the Mac version via steam. You won't be able to install from the disc, but if it save enough money does it matter? With the PC version it works out cheaper to buy the standard version and add the Babylon civ via steam DLC, unless you specifically want the making of video or soundtrack of course.