oddsod nailed it. your site is worth whatever someone else is willing to pay.
one rule of thumb I've heard is: take the site's annual profit, and multiply by three.
that's profit, not revenue. This takes into account the technology/marketing costs, and any staff or expenses necessary to maintain whatever it is that the site does.
When you're setting an asking price, start with a number proportional to what the site is currently earning, and maybe bump it up if it's sitting on a killer domain, or if it comes with a massive mailing list, piles of .gov or .edu backlinks, solid PR, other assets, etc.