There are many website that give an estimation about a website worth and profits. Just search Google for 'website value' or 'website worth'. My favorite is websitetrafficspy.com.

The closest I have seen is to research the company, say in Jigsaw, then look at the company revenue.

There is no accurate way of knowing exactly the profits of a website using sites like webworth etc. Most of the times they provide statistics that is not even close to the truth. The only way is to contact the owner and asking him. When somebody is willing to sale his website he will gladly give you all the statistics you're going to need.

Try Compete.com. They have a good traffic service to determine where everyone's traffic comes from - from there you can make rough estimations about what kind of volume people are doing and how much they're making.

I agree with Dave.. I like doing approximations. There is no 'exact' way to do it but you can do some pretty good approximations by looking at the proper data, figuring out what they're selling, and moving accordingly.

No tool will give you the exact profit that a certain site do make. The best they can do is give you an estimate and this is usually far off but having an estimate is better than nothing. :)