The Student Room Group
buying second hand is possibly best - the cheapest you can normally get them as new is 4.50 - thats for the CGP books, which are worth investing in.

I think revision guides are essential for science and ICT. When I did my GCSEs, I felt that my notes from school were sufficient enough to revise for History, English, Geography and German. You could save some money there by buying for fewer subjects. I didnt do business studies so cant advise you on that.
Reply 2
I got my resistant materials revision guide for £1 off Amazon.co.uk lol (second hand). If you buy used you can get them really cheap, the book was actually worth £4.50 :redface:

I don't see why you want second revision guides really, but different sources are good I guess.