I know this was £12 a couple of weeks ago at Gameplay, but this is the cheapest I've seen the digital distribution version. The offer ends on the 28th.
Batman: Arkham Asylum exposes players to a unique, dark and atmospheric adventure that takes them to the depths of Arkham Asylum - Gotham's psychiatric hospital for the criminally insane. Gamers will move in the shadows, instigate fear amongst their enemies and confront The Joker and Gotham City's most notorious villains who have taken over the asylum.
Batman: Arkham Asylum Download Features:
Investigate as Batman, the WORLD'S GREATEST DETECTIVE, by solving intricate puzzles with the help of cutting edge forensic tools including x-ray scanning, fingerprint scans, 'Amido Black' spray and a pheromone tracker
Face off against Gotham's greatest villains including The Joker, HARLEY QUINN, POISON IVY and KILLER CROC
Become the Invisible Predator with Batman's fear takedowns and unique vantage point system to move without being seen and hunt enemies
Choose multiple takedown methods, including swooping from the sky and smashing through walls.
Explore every inch of Arkham Asylum and roam freely on the infamous island, presented for the first time ever in its gritty and realistic entirety
Experience what it's like to be BATMAN using BATARANGS, explosive gel aerosol, The Batclaw, sonar resonator and the line launcher
Unlock more secrets by completing hidden challenges in the world and develop and customize equipment by earning experience points
Enjoy complete superhero freedom in the environment with the use of Batman's grapnel gun to get to any place you can see, jump from any height and glide in any direction