Cheapest around for the collector's edition that I can find. I think Zavvi just knocked it below £40 so you can't use the APRIL4 £4 off code. If you can find something for 60p you can knock another £2 off (kind of). £39.99+ at Amazon, ShopTo, Play, Gameplay and HMV.
Put in APRIL2 code at checkout.
Shopto's description (Zavvi, Play and Amazon don't actually say what's in the limited collector's edition):
Alan Wake: Limited Collector's Edition contents:
Alan Wake game
Exclusive in-game audio commentary and hints by Remedy
"The Alan Wake Files" - 144 page book that includes a short story written by Alan Wake and compiles the FBI dossiers investigating the events in Bright Falls, WA
Exclusive Audio Disc with soundtrack and score
Game Add-On Token redeemable on Xbox LIVE for the first Alan Wake add-on content pack
Exclusive Xbox Disc with special content: Alan Wake Xbox LIVE Theme and Alan Wake Avatar for Xbox LIVE