in the past, when trawling for reading material, i've found the cheapest option to be chariddy shops etc. does the ultra-intro-uberwebnet offer anything analagous?
I use Amazon Marketplace- just log onto to www.amazon.co.uk and put in what book/author/genre you want. It will give you a new price and a variety of second hand prices. All the second hand books I have bought have been in mint condition, very cheap and arrived very quickly (the very next day in one case!). Even brand new books can be found. Don't abandon the charity shops becasue they are competing in a tough market though.
If you want a particular book that's out of print, or otherwise hard to find, the 'abebooks' website is excellent. But just browsing, then I'm with you on charity shops. See what publishers are doing these days? Making bigger paperbacks, and charging you more. Same number of words per page, same number of pages, just bigger paper - and putting a fiver on the price! Huh.
...not to mention their practice of publishing new books in expensive hardback. If it's something I'm interested in, I've usually had it out from the library by the time the paperback edition appears, so serve them right!