I don't think there's a formal way to do this. After all, YouTube was valued at $1.6B but news internal to Google is saying that they're now stuck with it since they're not clear on how to generate revenue out of YouTube (real revenue, not few banner clicks). So how come it was valued at $1.6B? Only God knows. The price seemed right.
The following is just an idea (in other words, I'm pulling this out of my a$$):
A formula for valuing a website might consider the following factor:
1- Rate of growth in number of users (new registrations)
2- Number of daily hits.
Also, make sure that any numbers you indicate (especially the number of daily hits) can be verified by a 3rd party service. Such service can attest that n number of users are hitting your website every day. 
Disclaimer: I haven't done this before, but I'm suggesting ideas.
Good luck