Great question, Harry.
As I've said, the whole process is an experiment to me. I'm interested in a number of factors, not just reaching that end goal. (Of course, hitting that goal IS important, don't get me wrong.)
Going into this, my expectation was that the average individual would pledge anywhere from $1-5. After all, that's what the book is selling for elsewhere. This edition will be an enhanced one - deleted scenes, information on the writing process itself, an inside look at the sequel, etc. So the pledge would be getting MORE than what they could buy elsewhere for the same price.
Right now, the average pledge is higher because there are fewer people pledging. That average should go down as the number go up, unless people begin to pledge the higher categories in order to get the premiums that go with them.
I think most pledgers are doing so first because they believe in the concept of making good books freely available and second because of related perks/premiums.