Anecdotally, I'll pay up to $5 for a book on a whim if I think I might be interested. If it's more than $5, I download a sample and might by the book, if and when I even get around to reading the sample, and if I like the sample.

It doesn't matter if you have a great value proposition, without a solid sample I place it in the same bucket as unsolicited phone calls to lower my phone bill.