Leanpub books are bought using a "variable pricing" model. This means that the author or publisher sets both a minimum and a suggested price for the book, and then you pay what you want for it. Since Leanpub pays such a high royalty rate (90% minus 50 cents per purchase), many customers choose to pay more than even the suggested price for a book, to show support for the author.
When you buy a Leanpub book, you will see a pricing slider that lets you choose how much you want to pay. To pay less, move the slider to the left; to pay more, move the slider to the right. You can even just type a number in the box where you see the price.
You will also see a slider that shows how much the author will get, and you can use this slider to set the book's price. Where the author is sharing royalties with a good cause, you'll also see how much the cause is going to get.