Many believe the topic of Domain Appraisals and Values to be speculative as the ultimate value of any domain is based on the amount a buyer is willing to pay, and a seller is willing to accept for a domain. Certainly the more descriptive the term the higher the value tends to be, as well as the length of the domain (shorter is better), and the extension.
That being said, there are many sources available that will offer appraisals for a fee and will claim varying degrees of "science" behind their valuations. Any domain appraisals should be considered as a guideline and used in conjunction with one's own experience and relative to offers previously received for the domain.
For many domain sellers the question becomes whether or not it is more important to sit on the domain and wait for a potential sale, or to set a reasonable price that is more likely to initiate interest and provide them with instant cash flow to support and increase their business.