A pirated copy demonstrates a wish for the product but does not neccessarily demonstrate a real market with a real demand for it at that price point.
For example, if you look globally, there are huge populations that have the capability to play pirated games but (a) can't consider $20 as affordable; and (b) can't use western payment infrastructures such as credit card payments.
If you look locally, a person who would play World of Goo for free but if offered it at $20 would choose some other entertainment - that's not a potential customer, that's shouldn't be counted in demand.
When I investigated a micropayments startup (we wanted to try it in our country), we found some surprising facts - a huge % of the world's population don't have access to banking systems, even if they DO have some disposable money and want to pay for online games - non-cc alternatives to buying Facebook currency and such were huge requests when we did our customer research.
I don’t know about you, but for €15, I could enjoy 10 hours of films in a cinema and back then™ when I was playing and buying games, I got a few months out of a single game (Pharaoh and Caesar III come to mind). 2$/h is definitely not a ‘fair price’ for a game I have to run on my own computer.