a lot of people say 6 months revenue from a stable site is the value, but i've seen guys sell sites on ebay for a LOT more. if the site doesn't have stable traffic or trades (depending on what kind of site you have), then it's often worth 3 months revenue.

if a site with exclusive content, you must figure out what the content is worth. if non-exclusive transferable content, then just add in the value of the content to the revenue for pay and premium avs sites.