GameAgent takes 75% off a selection of BioShock for Mac downloads, with prices starting at just $5. (You'll need an account to view these games. It's free to sign up.) That's the lowest price we could find by at least $10. Titles include: BioShock, BioShock 2, BioShock Infinite, and BioShock Infinite Season Pass. A Steam account is required to download these items.