dvd.co.uk is selling "Mario & Sonic At The Olympic Games" for the Nintendo Wii for £29.89 inc Del.
You can get cashback through the usual sites (e.g. 4% from Quidco making it just £28.69).
This is the best online price. You may get it cheaper from Tescos instore but it seems a bit hit and miss whether people have been able to get it or not from their local store (see link below).
In Mario & Sonic at the Olympic Games, players compete in events that take place in environments based on the official venues of the Beijing 2008 Olympic Games. Using a supporting cast of characters from the amazing worlds of both Mario and Sonic, gamers can compete as or against a range of lovable personalities including Mario, Sonic, Luigi, Knuckles, Yoshi, Tails and more. Innovative usage of the Wii and DS control systems to maneuver a favorite character allow players to race the likes of Mario and Sonic down the 100m track, engage in exhilarating rallies in table tennis and churn water in a swimming heat, all while competing for the much sought after gold medal.