CDKeys offers Steam download preorders of Batman: Arkham Knight for Windows, Mac, and Linux bundled with the Harley Quinn Story Pack DLC for $20.99. "Like" the CDKeys Facebook page to receive a unique coupon code to drop it to $19.94. (There's a Facebook link on the top of the page.) That's $2 under our mention from two weeks ago and the lowest price we've seen. (It's a current low by $16.)

Note: It's scheduled for release on June 23 on PC, and in the fall for Mac and Linux.