Get Games offers downloads of Batman Arkham City for Windows for $4.59. According to the GameTrailers, this version will automatically be upgraded to the Game of the Year Edition. Assuming you get the upgrade, that's $3 under our mention from two days ago of the GOTY edition (which included Mac downloads) and $3 less than the lowest price we could find today for the GOTY edition. It's also the best price we could find for the standard version by $25.

Update: Note that Steam is required to activate this game. The key you receive will also work for Mac.