Appraisal of a domain name is a very tricky project, and it will vary depending on the type of buyer you are. Many are concerned with current income (parked, or other) in order to determine the value of a name. I've seen countless names that sell for much more than you might imagine just by the name alone, because they get a little traffic and are earning a decent amount. Some names just say the right thing, and the buyer wants it. To get a good picture of domain prices, you should look at DNJournal's Sales page for the non-auction/backorder domains. Domains that sell that aren't caught up in a bidding frenzy.
http://www.dnjournal.com/domainsales.htm
Another place to look are the Auction sites like Sedo and Afternic. Take a look at the Recent Sales.
http://www.sedo.com
http://www.afternic.com
I like to use URL Trends to get a quick overall picture of a domain when I start to research what it's value might be, before I make an offer.
http://www.urltrends.com
At the end of the day that's why the saying usually goes.. It's worth whatever someone is willing to pay for it.