Downloading for free is not morally acceptable for me, I would like to support this business plan.
"Because everyone in the negotiation is a businessman and any number we are mutually happy with is a morally acceptable number."
I buy tons of books. If the author makes the book available at a choose your price, then I always choose the lowest price even if that price is free. If it is morally acceptable to the author, then it's acceptable to me. I've bought books for as low as free, but others for more than $100.
This is quite different from stealing or pirating a book. But you should do what you think is right and leaves you with a clean conscious.
Variable pricing is really an interesting business model. I have three books for sale on leanpub, each with a suggested price of $6 with a minimum of $4. A lot of people pay more than $6, BTW, the most so far that anyone has paid is $50. Payments in the $10 to $12 range are common, but probably less than 15% of sales.