There are many tools to determine a names worth but unfortunately no formula's.
NamePros is a great tool. Read everything on this forum. No exceptions.
The most common tool is Google adwords keyword tool. Start there. Try DNJournal .com and run the high dollar sales into the keyword checker and see if you can find a correlation. You won't find one but it will help you realize how difficult assessing a name really is.
Search volume, CPC levels, backlinks, trends, industry, SERP competition, product vs non-product, all of these factors play roles in assessing value. And I'm sure many more. Almost forgot extensions...it's late.:zzz:
Read the forums and ask lots of questions. Most here will help.