ebay - usually go for £25-£30
Even if you have to pay full price they are SO worth it - and this from someone who often spends £40 in total on Christmas/birthday presents. DS got his for his 2nd birthday (after much agonising about its cost) and has used it nearly everyday for 3 years. It has been the most cost-effective present ever. So much so that after much more agonising we have just taken delivery of the next one up for his 5th birthday in Nov. It is double the price but should last until he is 12.