What is the dearest gardening book?

The dearest book I have found is:

Flora Europaea (Flora Europea)
T.G. Tutin (Editor), V.H. Heywood (Editor), N. A. Burges (Editor), D. H. Valentine (Editor), S.M. Walters (Editor), D. A. Webb (Editor)
(April 1980)
Publisher: Cambridge University Press
ISBN: 0521232058

Amazon.co.uk were selling this book for £550 on 18 May 2004 Flora Europaea (Flora Europea)