The best way to value domain names is by using comps, as does the real estate industry. Variables to consider are:
1) Industry
2) Name type (one word, two word, hyphenated, tld, etc.)
3) Traffic metrics (e.g. type in traffic)
4) Type of traffic (high valued age group, country, etc.)
5) Age of domain registration
6) Ability to develop a site that can be monetized
Two resources for finding comps are DNSalePrice.com and NameBio.com. Find similar names with similar characteristics and you can get a good comp. It takes time.