CDKeys offers Steam download preorders of Batman: Arkham Knight for Windows 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 $2 below our mention from two weeks ago and the lowest price we've seen. (It's a current low by $28.)
Note: It's is scheduled for release on June 23.