Tools like http://dnscoop.com provide an automatic estimation on the value but the value of a domain is really whatever you can sell it for, but the basic standard is the more backlinks ie higher pr the higher price you can ask. Also if its a dictionary word you can ask a pretty high price.
I really believe the answer to that question is how much someone is willing to pay for it. I believe the sites that estimate domain value are pure garbage. You might have a domain with no PR or traffic that someone has come up with an awesome idea for, and they may very well be willing to overpay you for it.
Domain value estimation..
Linear Mode
