There are lots of ways to get an appraisal on a domain, and very few are very accurate. Any free appraisal is essentially useless because a domains value is based on human factors (how easy is it to spell, is it memorable, does it make sense) and those are almost impossible for a computer to accurately define. The free appraisals are all an automated script looking at a string of characters and trying to figure out what they say (if they even go that far) and basing a price off some very basic criteria. Those scripts look at how many characters are in the domain, if those are numbers or letters, or both. What TLD the domain is (com vs net vs org, etc.). How many words are in the domain (if it can even tell). Those are all minimally useful, but it takes a person to look at a domain and know if that would make a good brand. When it comes to figuring out the value of a domain, there is no substitute for an experienced person evaluating the domain by hand. One source that will give an appraisal and reasons for the appraisal is Valuable Web Names.
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Knowing what a domain is worth is tricky because there are a lot of factors that go into it. I’d recommend contacting VWNames.com to get an estimate, you can do a basic appraisal there that will give you a good idea of what the domain is worth. Ultimately, though, if a seller is stubborn then the price is theirs to set.