OK, so this is in store only and may be store specific (I haven't checked), but Oxford Street HMV is accessible to alot of people, and they were just wheeling out a big stack of them.
ROCK BAND Drum Kit for the XBOX 360 for only £15. I don't have one, but have seen them elsewhere for at least twice that.
play.com = £38.99
amazon.co.uk = £29.99
zavvi = n/a
thehut = n/a
This is the standalone drum kit without the game.