A lot of people use those automated domain appraisal sites and services but I don't think that those carry any merit at all. If you want a real appraisal, you will have to use a paid service, but I also think that those are usually quite inaccurate as well. Any domain is only worth as much as someone is willing to pay for it. A lot of domains sell for a lot less that what you might expect and some sell for a lot more as well. It is hard to tell and best to do on a case by case basis.