Sites online are not the best places to get the evaluation of the price of an old book.
Your best bet, if you're unable to do so yourself, is to take it to a rare/antique book dealer and ask them to appraise it for you.
How much a book is worth depends on:
-Condition: Mint, fine, good, bad, poor. Mint books are worth a lot more than books in "fine" or "poor" condition.
-Does it have the original dust jacket/cover, and what condition is it in?
-How many copies exist?
-What edition is it? First editions are much more valuable than say..books from the third or fourth printing.
-Is there anything particularly unusual about it? Doe it have an author's signiture, a vellum plate, an (intact) map?
The first thing you'll want to look at is the condition of your book. Is the spine cracked? Are there waterstained pages, or has it been damaged by mildew? Is the cover separating from the pages? Are the pages dog-eared or are there sticky things or stains (other than water) on them. Are the covers closing properly and is anything flaking off the book itself? Has it been patched or repaired with librarian's tape, is there discoloration on anything at all?
Next, check the publication information for a publication date, the publishing house, etc. Check to see when the book was first published.
After that, check for anything unusual. Books that come with intact maps (no rips, tears or pieces missing, very little wear on the edges) that belong with them (such as railroad propaganda, geographical books) are worth more with them then without. If there's an unusual picture (on vellum or on regular paper) make a note of that. Do as much research about your book as you can.
Check to see what the condition of other copies of the book you have are. If most of them are in poor condition, with a few that are good or fine, and you have a copy in fine condition, then it's likely people will be interested in your book.
Is it fairly unusual as a book in and of itself? Check to see how many copies are still around, see if it was ever mass marketed. Mass-market books can decrease greatly in value because everyone has one.
Even after you do all the research and checking, it would be a good idea to take it in for an appraisal by a rare or antique book dealer, just to get a second opinion. If you overvalue, no one will buy it, and if you undervalue, you may regret it later.
I hope this helps you, and good luck with pricing!