Valuing a website is a tricky business.
Stating the obvious: A web site is only worth what someone is willing to pay for it.
Most valuation sites try to put a value on sites based on a number of factors that might, or might not be relevant. These can be traffic and other analytic numbers. Some will also take the name into consideration. The biggest factors (in my opinion) for driving a value for a site are (1) the site's history at driving revenue, and (2) the site's interest to a specific person.
If a specific person is interested in a site, then they are likely to pay more for it. For example. John Doe might want a site with his name, so he might be willing to pay a premium for the www.johndoe.com site. Similarly, a company like State Farm might be interested in a domain like 'insurance.com', so they might pay a premium for it; however, those premiums would be likely limited by the amount they believe they can drive from it.
As such, the biggest factor in the value of a site is how much revenue it is currently driving. If you are not driving thousands of dollars a month from your site, then chances are, it isn't worth tens of thousands of dollars in the open market.
I also tend to see forums as being worth less that sites. If you have a strong forum, with a very active community that engages in dialog, then there might be value, but the issue comes back to... are those people buying things or clicking on ads? If not, then you can have millions of people, but they are worthless to most people that would spend money on a site.
Same with a site - how much content is on it? How dated is the content? How relevant will it be in 12 months? I'll guess that information on celebrities tends to date quickly. Thus the value of a celebrity site drops quickly if it is not maintained.
Lots of factors to consider. If you have high traffic, then you might talk to companies that would be looking for the type of people visiting your site. They might offer you something for the site to gain that traffic.
As someone said - if you aren't monetizing what is on your site, then the value is closer to $14.99 or whatever you pay for a domain name.
Everyone thinks they have the next big name. The problem is... there are often a dozen other sites and similar names out there that could be used as well.
(the opinions expressed in this post are my own and do not represent my employer in any way)