The minimum price for the game is $1.00 and works for Windows, Mac, and Linux. Comes with a DRM-free copy and a Steam activation key.
It looks like you get some digital bonuses if you beat the average price (currently $2.62).
Digital Soundtrack (MP3/FLAC)
Bastion Digital Art Pack
Bastion Sheet Music
Bastion iPhone/Android Ringtones
Additional bonuses if you pay $25 or more.