Considering Kindle books are probably hosted over s3, those costs are relatively marginal. Amazon charges third party S3 usage for storage/transfer in cents/gig

Considering Kindle books are probably hosted over s3, those costs are relatively marginal. Amazon charges third party S3 usage for storage/transfer in cents/gigabyte, presumably at some appreciable margin. Their revenue from a single 0.99c purchase can probably cover costs related to a copy of that one book, plus another four or five free ones, for a year or more. Even better if they have a single "canonical" copy that is encrypted for your device on the fly at download time.