There's more to it than just ad rev. You have to consider the domain name worth as well (among other things) and a key consideration beyond ad rev for determining if you should even buy is how the site gets its visitors (direct links, search engines, Google only etc). You'd prefer diversity there and to not be potentially bent over a desk by Google.

If you're not the site owner, Google won't share stats on the site but there is a site that can estimate that sort of thing for you called Alexa.com. Here's FTR on Alexa.

Flopturnriver.com Site Info