Newegg offers Steam downloads of Batman: Arkham Knight for Windows for $4.99. That's tied with last month's mention and the lowest price we've seen. (It's the best deal now by $15, outside of the mention below.)
A close price: CDKeys charge $5.79. It also offers the Premium Edition (which includes all DLC packs) for $6.49. New customers can provide an email address on the CDKeys Facebook page to get a unique coupon code that cuts an extra 5% off.