As I said in a previous Raspberry Pi thread[0], the comparison is supposed to be with high / school school textbooks, not college ones. School textbooks are cheaper than those used at university.

Raspberry Pi is a charity "to promote the study of computer science and related topics, especially at school level" and the device is aimed, in part at least, for use in teaching programming to children.[1]

$25 (roughly £15 at current rates) is a good rough price for a school textbook in the UK. A quick Amazon search shows this.[2]

Bear in mind, too, that the product is aimed in part at developing countries, where school textbooks may be cheaper.

[0] http://news.ycombinator.com/item?id=2949932 [1] http://www.raspberrypi.org/?page_id=2 [2] http://www.amazon.co.uk/school-textbooks-Books/s?ie=UTF8&...