Keep an eye out for second hand games on Play.com and Amazon... Sellers seem to keep making mistakes. I can now reveal I got Mirrors Edge and Killzone 2 for £1 each inc postage because the seller made a pricing error, and just honoured it (because they were a company rather than a seller, selling on one of the above sites).
Managed to get Silent Hill Homecoming for £3 inc postage too last night, again a pricing error which the seller is honouring!