httpwebwitch - 1:35 am on Mar 25, 2012 (gmt 0)
first of all, ignore all the automated sites out there that will give you a valuation based on your Alexa score. They're complete rubbish.
One rule of thumb I've heard many times is: take the annual gross revenue the site earns, and triple it.
But the real answer is: whatever they're willing to pay!