Is there a web site that gives a guide on what a particular old book is worth.
Rather like what "Artprice" does for paintings

Search with   antiquarian    and you will get very many sites.

This German site:  abebooks.de
says it offers links to 13,000 antiquarian sites with 70 million books.

If it doesn't come up in English or your screen, here are the items you can enter:

author
title
publisher and or year
key words
ISBN
Country
           first edition   signed
Price in Euro     min.       max.
sorted by     price increasing    price decrease
search only from German sites (click off)

At the right then hit  "Suche starten"

"Zurücksetzen" may save a search, I don't know.

Good luck, Myoarin

abebooks is also available in english at abebooks.com. You might also
try the advanced search at alibris.com.

abebooks is also a uk website. I worked in a bookshop in australia for
a year or so - we used biblioquest - i think it works on a world-wide
basis.
tried amazon? you never known.
also, i would say try www.cbfol.com - this will tell you if the book
is still in print - not quite what you necessarily want - and also
give publication details

You can search http://www.byblos.uk.com. If they don't have the book
you are looking for try http://www.bookfinder.com