This is a deal I saw in my local Blockbuster, an official poster so I assume it applies elsewhere as well: -
"get Forza Motorport 2 for £9.99 when you part exchange one of the following games: -
Brian Lara 2007, Champ man 2007, crackdown, def jam icon, fight night round 3, gears of war, lost planet, tom clancys advanced warfighter 2, tom clancys rainbow 6 vegas, virtual tennis 3 (xbox 360 games)"
If you go to this link: -http://www.hmv.co.uk/hmvweb/displayProductDetails.do?sku=448445&tduid=f6157bb3acb86ffb8af4c564fc4f9f7b
you can get Fight night round 3 for £17.99
so you take that in and pay £9.99 so it costs you £27.98.
Obviously cheaper if you already have one of the games and/or if you get the game from ebay..
You can do the same with other games, like Spiderman 3, or UEFA CL (both 30 quid, get Fight night and trade in, save a couple).