According to find-games.co.uk the next best is £25.85 at Shopto.net.
Although a clearance item, there appears to be stock available at quite a few stores local to me.
Rock Band 2 lets you and your friends take your band on an even more expansive and immersive world tour, in person or online, and continue your rock and roll fantasy. Harmonix, developers of the smash hit Guitar Hero and Rock Band franchises, deliver Rock Band 2, the next step to the platform that lets audiences of all ages interact with music in an all new way. Rock Band 2 lets players vicariously jam out as some of the best guitarists, bassists, drummers and singers of all time. Rock Band 2 delivers the most authentic and ambitious music game to date - by challenging rockers to master lead guitar, bass guitar, drums and vocals with a polished, streamlined, and amazingly fun and challenging experience. Rock Band 2 features a track list that puts all other music games to shame with more than 100 ondisc and free downloadable tracks from some of the most hallowed bands of the rock pantheon.
For ages 12 years and over.