Something is only worth what someone is willing to pay for it. Given that Facebook was valued at $15b by Google you can be fairly sure that all of them are out of both yours and my price range to buy. Yahoo Answers would be heavily subject to if you were buying the rights to keep the name or not and if you were what the licensing fees/ conditions to the Yahoo name are. For all of them you would need to look at how well they are monetised at the moment, is there a natural fit to your existing portfolio of products/ services/ sites and how likely is it that your competition would want to buy them.... that added to the traditional Goodwill elements would make up the majority of the price