Production Cost Not Linked To Book Price
The production and distribution cost of a book has a very small impact upon how much it is sold for so expecting a digital book to be much cheaper just because you don't have those costs is wishful thinking.
The vast majority of a books cost is so that any advance to the author can be got back plus the high costs of editing, marketing and support for literature in general. Those costs are still there no matter what format the book is in.
Most writers don't earn enough to make a living out of it and if digital books get a lot cheaper then expect there to be a lot fewer professional authors out there. Cheap / free music means that for a lot of musicians they don't make much on the actual albums they sell but instead have to get their money via touring and merchandise; authors don't really have that option.