We've purchased one of these from TJ Hughes today for our son's birthday. It is the cheapest we've come across so far. They are currently £29.99 in Toysrus and £39.99 on Amazon.
£20 from TJ Hughes, they are available online aswell but don't forget to add extra p&p costs.
Would make a lovely Xmas pressie for the kids, it is a mini sized keyboard with 37 keys, 10 tunes, 30 patterns. It also comes with a free Mic Headset.