Peter from Leanpub here. If you're down to your last $60, please don't feel guilty not paying anything! Come back later and buy the book (or one of raganwald's other books) if you want. (On the bestsellers page, his other books are currently in the #5 and #9 spots.)
The reason we did the sliders this way is to encourage people who have extra disposable income to pay more. Since the Leanpub royalty rates are so good, we felt we should show what the author gets right on the purchase page. That's why we subtly change their color as they are dragged up and down, as well as live-updating the royalty, etc. So I'm really happy since this shows that they are working as intended :)
It would be great to have the ability to post-pay. I can imagine finishing the book and realizing that it was worth $20, and wanting to pay more. In this case if I come back to LeanPub there would be a list of books I selected to postpay, so after I finished reading them I could judge the true value and pay that (or add to what I have already paid).
Not sure how well that would work in practice but it resonates well with me. What hooked me was the title and the word free, the advantage with free is that I don't have to make up my mind on how much it is worth. My first reaction was that I could, since it was free, just grab it and see what it was.
But the instant I came to the site I felt guilty. I ended up paying the suggested price only because the suggested price was sensible (if the suggested price had been more I'd probably would have paid less) and because leanpub seems awesome (just found out about it). Since I know nothing about the book itself I didn't take that into account when buying, it could of course be worth much more.
EDIT: DRM-free and giving me multiple formats to choose from (and not forcing me to select one) is awesome and is one of the reasons I really wanted to buy it. Otherwise I would have had to strip the DRM first (I don't accept DRM on e-books that I buy, even if it makes me a criminal).
Combine it with other features (Write the author, discuss the book with author and readers, rate the book) and invite the reader on the last page of the book.
It worked very well. Knowing how much the author gets and seeing it go up and down makes me feel empathetic and makes me think how I'd feel if I were the one selling. I always find that when payment pages are set up this way that I'm not being "sold" something but instead like I'm being offered something useful. It also makes the whole thing feel more human if that's the right word. Payments are almost like adversarial type situations where one party is trying to take money that you're protecting. When it's set up like this it helps break down that attitude and shows you how both you and the seller are helping each other.
The reason we did the sliders this way is to encourage people who have extra disposable income to pay more. Since the Leanpub royalty rates are so good, we felt we should show what the author gets right on the purchase page. That's why we subtly change their color as they are dragged up and down, as well as live-updating the royalty, etc. So I'm really happy since this shows that they are working as intended :)