You must consider how long the domain name is and the extension of the domain. A .com extension will likely be worth the most because it's the most known extension and is easy to remember for customers. Obviously the longer the name or more words in a domain will be harder to remember, making it's value going down.

The higher the domain ranks in the search engines the more likely the web site gets a large amount of traffic. But this depends on the keyword or keywords it ranks high on, for example, the keyword music is going to get more traffic than the keyword webmaster.