BlahDVD is selling "Cars Mater-National" for the Xbox 360 for £17.99 (inc Del.)
You can get cashback on top of this from the usual sites (e.g. 8% Quidco, making it just £16.55)
Next best price seems to be £22.99 from HMV.
Cars Mater-National features an all-new International group of racers vying for their chance to compete against Lightning McQueen in the First Annual Mater-National Race Festival, hosted by Lightning's best friend, Mater. Explore the new and improved open world of Radiator Springs, play in the all-new Monster Mode, jump into the fast lane with 6 new International racing competitors, burn rubber on new tracks, and twist and turn your way through new mini-games like Ramone's Rhythmic Rumble and Fillmore's Fuel Frenzy.
- Explore the new and improved open world of Radiator Springs
- 20 challenging mini-games
- 5 exciting new international characters
- Monster Mode
- Over 10 multiplayer games