Google has enough information to work this out: via the Gmail cookie they can establish identity in a way that persists over time and web client. From there, they can figure out what ads I have clicked on. (And of course they know how much this click is worth to them.) They also know what searches I perform and how frequently, and so can take a guess as to how much Google is worth to me. (They may even know the value of my content to them (via Google Analytics), though the analysis probably gets a little tricky here.)
Do they do work out this figure? How much could they profit from it?