ChoicesUK are selling "Ridge Racer 2" for the Sony PSP for £4.99 (inc Del.)
You can get cashback through the usual sites (e.g. 5% Quidco, making it just £4.74)
The next best price seems to be £12.00 from Zavvi.
Perfectly suited to PSP, Ridge Racer 2 is easy to pick up and play, keeping the high speed drifting fun in the palm of your hand and letting you take this supreme racing experience with you wherever you go.