You receive value from consumption of the book.
Have a mechanism for ebooks where you pay a much smaller fee when buying the book (maybe $3 of the total price of $10 for the book), then only micro-pay for pages you actually read in that book up to the whole price of the book.
A revenue stream and to filter who can comment.
Have a mechanism for people to comment online (on a blog post or an article) only if they pay a small amount.
As a good measurement of what was the right answer.
Allow users to donate how much they want (maybe even less than 1 dollar cent), based on their satisfaction of the result they got in the SERP. This will give a better indication as to what is a really useful answer to a specific query.