centipede - 6:34 pm on May 21, 2011 (gmt 0)
Fair Market Value is the price which would be agreed upon between a willing and informed buyer and a willing and informed seller.
No stupid website valuation tool - which know next to nothing about the reality of your site's costs and income - can tell you that. They're just playing guessing games.
Like @httpwebwitch said: 3x annual profit is a good number for most sites. Plus or minus a bit.