The pricing of a book is an interesting economics problem. For economic efficiency, the price should be equal to the marginal cost. But as often happens, marginal costs are sometimes far below average costs (due to fixed costs.) So P=MC leaves a deficit in the recovery of full costs. Therefore P>MC has to be it. For ebooks, the MC=~0. But if you shift some of the deficit by pricing ebooks > 0, you reduce the deficit. I have been thinking about book pricing because we have to attach a price to my book.
For hard copies, the price has to reflect the cost of production and distribution. Actual printing costs are between $1 to $2, with the lower figure for high printing volumes. Shipping and handling costs add up to another $1. Some books have to be distributed for free — to reviewers mainly — which adds to the fixed costs. A reasonable price close to the average cost is Rs 200, and for the US, $5.
For soft copies of the book, there are two avenues of distribution. One, as a Kindle book from Amazon. Two, as a PDF download from the book website. Kindle distribution is whatever Amazon charges and a little for fixed cost recovery. I suppose $3 would be an appropriate price since the cost of printing and S&H is not incurred.
For PDF download, I think there has to be a mechanism for people to donate whatever amount they feel is appropriate. The marginal benefit from the book depends on the reader. So if you buy the book and find it makes sense and you are glad that you read it, you could try to estimate how much you value it and decide to donate that amount. For someone it may be a positive number, zero, or a negative number. Since you cannot donate a negative amount in monetary terms, I suppose the lower bound is zero.