Was browsing in the GAME store in Westfield, London, today when I noticed they had an offer for Mass Effect 2 on the Xbox 360. Not sure if it's nationwide but certainly worth a try.
You can get the "Game of the Year" for just £4.99 when you buy another other item in-store. I found some cheap Toy Story related tat for £1.99 and got the whole thing for £6.98.
GAME had the title pre-owned for £12. So I got an absolute bargain.
You'll notice on the receipt pictured that they make reference to Need4Speed and Assassins Creed. I assume the original offer let you buy ME2 for this price when buying one of those titles. Seems like they've now expanded the offer to cover anything in-store.
So yeah, basically you can get ME2 for about £7 with a bonus piece of tat.
Alternatively buy something like N4S and ME2 and get them both for about £40.