An audiobook is usually the most expensive edition a publisher makes, despite having no paper, printing or shipping. The costs sit almost entirely in the recording process.
Recording time far exceeds listening time
Producing one finished hour of audio takes several hours in the studio, because passages are re-recorded for errors, mispronunciations, noise and delivery.
A full-length novel therefore represents days of booked studio time, and the studio is paid for the hours used rather than the hours that end up in the file.
Longer books cost proportionally more, which is why some titles receive audio editions and others of similar sales potential do not.
Studio requirements are also strict, since ambient noise that a listener would notice on headphones has to be excluded from the room entirely.
Narration is skilled work paid accordingly
Narrators are typically paid per finished hour, at rates reflecting a performance that must remain consistent in pace, tone and characterisation across many sessions.
Established narrators command more, and publishers pay it because listeners follow particular voices and a familiar narrator supports sales in the way a familiar author does.
Full-cast productions multiply this cost, since several performers must be scheduled and directed rather than one working alone.
Preparation and post-production are substantial
Before recording, someone marks up the text for pronunciation, character voices and emphasis, and researches unfamiliar names and terms.
Afterwards the recording is edited, corrections are dropped in, breaths and noise are removed, and levels are mastered to the specifications each retail platform requires.
Proofing then involves listening to the entire book against the text, which takes at least as long as the finished audio runs.
The costs are fixed regardless of sales
Almost all of this spending happens before a single copy is sold, and none of it scales down for a title that sells modestly.
An ebook of a backlist title can be produced for very little, while an audio edition of the same book requires the full production cost again.
Authors producing their own audio face the same arithmetic, which is why many use royalty-sharing arrangements with narrators instead of paying fees upfront.
What the economics decide
Publishers commission audio for titles expected to earn back that investment, which concentrates audio availability among bestsellers and well-known authors.
The gaps in what is available as audio are therefore a map of commercial expectation rather than of what listeners would want to hear.