I think you'd have to pay about $2,500 to an unknown narrator to narrate the average novel. I think that's cheap.
As to how much a "successfully published author" would charge? Should be more if they can do it. Many would be poorer than decent text to speech. A totally different skill to either writing or acting (only some actors are any good at it). It needs practice. Also what you hear on a commercial audio book is not the work of a single person, even if only one is reading, and is likely edited.
Google/Alphabet hopes to get their text to speech good enough to automatically produce audio books from text, and then sell these giving the author a percentage (but only because the material is copyright).
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