Bit of an esoteric deal for sure, but those in the know will recognize greatness when they see it! As a comparison, the cheapest decent rival would be an Akai LPK25, which goes for £40 on Amazon. I actually found the build quality of the Rockband Keytar to be considerably better than the Akai, the synth-action keys have a much better feel to them, too.
As I say, probably for a limited audience but it really is a cracking deal.
Bear in mind that the output is a fully-fledged 5 pin MIDI DIN (not usb) so you'd need a MIDI-> USB adapter or other interface for connecting to your computer but it would probably still work out cheaper.
More info here: http://cdm.link/2010/10/hands-on-rock-band-3s-keytar-a-surprisingly-serious-80-midi-keyboard/