The minimum price of the book is free. You could grab a copy and determine if you like what you see and then pay the full price.
That there are tasks out there that some ought to do for free if someone else thinks these should be done for free?
The "pay what you think it is worth" model is not a scalable and viable approach. It most likely only works when everything costs money and "pay what you think" is a novelty that gains sympathy and attention.
But as soon as the novelty wears off it is not a sustainable approach.