A good starting point is to evaluate if the domain has “legs” in advertising terms.
For instance a domain targeting “Resort” or “divorce” has a good potential advertising income.
Maybe 1,000 attorneys paying $500 year to be listed in a heavy traffic site means 500K year.
But there’s more in terms of SEO, PPC potential etc.
Try this source a cup of weeks and you’ll have an estimate of your domain name. http://www.dnjournal.com/domainsales.htm