As background, I managed a publisher for 5 years and I have owned a publishing company (RockBench Publishing Corp.) for 6 years. Very few people make money on their books. I have written four, and have made money on two of them, but even that is rare. In most cases, you write a book because it gives you a bigger platform. So when you answer this question for yourself, don't think about the money you make from the book itself but from the opportunities that the book provides. In that context, the rule of thumb is to give away one book for every two books that you sell. But it's not quite as simple as that, because money is the currency of respect, and there's very little branding value in giving away a free book. You get tons of mileage, though, from giving away a book that they would otherwise have to pay for. So here's the answer: price the book normally, and then give away copies.