I know that most of you probably know this already but ..
Ebay is great for cheap games, just look at the ones that are nearly ending and you can pick up some great bargins, picked up about 5 games so far from ebay and not paid more that £30 for all 5.
Titles such as :
Tiger Woods 09
Dark sector
NBA09
Bourne Conspiracy
Top Spin 3
I know some of them are average games but 5 for £30 ain't bad at all
Also if its new games your after then try
http://www.123pricecheck.com
Select Games and then you game title and it will let you know where the cheapest games are.
Like i said most of you probably know this but for those that dont.
Many Thanks