CDKeys offers Steam download preorders of Batman: Arkham Knight for Windows, Mac, and Linux bundled with the Harley Quinn Story Pack DLC for $22.99. "Like" the CDKeys Facebook page to receive a unique coupon code to drop it to $21.84. (There's a Facebook link on the top of the page.) That's tied with our mention from last week as the lowest price we've seen. (It's a current low by $14.)
Note: It's scheduled for release on June 23 on PC, and in the fall for Mac and Linux.