You can just check how much people are selling smiliar books for on Internet, in web sites like eBay, Amazon, or Lelong. But here's a compilation of articles about how much your book is worth at a glance, just in case you fancy some know-how.
Additional read on second hand books' impact on the market:
My opinion? I don't sell my books. I'm a book collector, not a book dealer. I buy a book because I'm interested in its content. I might want to read it again ten years down the road.

But I would sell a copy of Gutenberg bible without skipping a heartbeat.