I stumbled upon a complete in box copy of Prey for Mac, and the key can be used on Steam to activate the game and have it added to your account. Well, you'd get a PC version, not a Mac one.
I'm looking to get a copy of Ghostbusters: Sanctum of Slime on Steam in exchange for it. Or, if you want the whole box, I can send it to you and you can play the game on your Mac, too, since it's an universal binary and it should work on Intel Macs.
PM me if you're interested!