Cheapest i've seen it i think...
Note this is the original version, not the game of the year edition.
The only difference between the two versions is the FREE DLC survival pack comes packaged on the GOTY disk, rather than having to download it for FREE from xbox live.
The download is 101MB
Asda: £23 (instore only, but posted 6 weeks ago, so might be out of date now)
Coolshop (formerly softUK): £27.86
After a brief look the cheapest i could find the GOTY edition is £39.99 at play/game/gamestation
Note: I haven't bought from this site personally, but they seem a legit company based on the comments from all their other deals