The last part of your post is known as the Drake Equation (http://en.wikipedia.org/wiki/Drake_equation). I'm guessing you've probably heard of it, but it could be of interest to other people, so I figured I'd link it.
Tangent: the Drake Equation seems to be the template for a class of "Google interview" questions: estimate something you can't really know, at least off the top of your head, in which you then start to identify factors, and start to guess at upper and lower bounds, to come back with a plausible answer, or at least a range. I suppose a lot of research is like that, but the Drake just happens to deal with factors that are harder to measure.
