It's a very inexact science. The fact is that none of those books listed have sold at the price quoted, so it could be argued that they are not worth even £27. I'm not sure which particular AGA cookbook you are looking at, but I have found a copy of Rosemary Moon's 1998 book on Addall for 98p.

Many booksellers are blissfully unaware of the intrinsic value of their stock. When they go to list it, and find no other copies listed, they quote a silly price. Then others come along and gradually undercut, but the old listings remain. All this time, nobody has bought a single book.

The situation is even more complicated on amazon, where the use of software to adjust prices against competition is widespread, so if there are only a few copies of a book listed, the prices bounce around all over the place.