Here's an example. There is no "market" for beautiful views from buildings. However, economists can use "hedonic pricing" mechanisms to get a sense of how much of the value of a given residence is attributable to its view. By the same token, information economists may want to try to measure the value of "open" vs. "closed" cultural artifacts.
For example, why are so many people adopting "wikipedia?" How much are they saving by using this free, P2P resource rather than, say, paying for a subscription to Brittanica or Encarta? Rishab Ghosh is pioneering such a "substitution method" of valuation. Moreover, we might want to assess human capital formation: what critical thinking skills are users gaining by both editing and assessing entries? Nissenbaum & Benkler suggest there are many capabilities enhanced by such projects.
The traditional story here is that new, P2P alternatives are going to kill off traditional information sources. However, even if that is the case, we need to be able to assess not only the costs, but the benefits, of the transformation. Moreover, I think one can tell a happier tale of symbiosis, or at least mutualism.