Try here: http://www.domainvaluation.com Or ask on this forum, people here are usually quite accurate valuing domains.
Most people calculate that by multiplying the sites 1 month earning X 12 but it may worth more than that depending upon the site theme as well as domain name.....
Most appraisal services are inaccurate. The only way to be absolutely sure is to put it on auction. Of course, you'd have to sell the domain then.
That would make the presumption that the site has revenue. If you want an appraisal, post in the appraisals section here or on other forums then it'll give you an approximate value