Taking your example of the Harry Potter audiobooks, I looked at the prices directly from the publisher, Bloomsbury Publishing. I compared the length of the paperbacks to the cost of the audiobooks, and it turned out that the trend was almost perfectly matching.
If we take the length of the first book as 1, and the price also as 1, and measure relative to that, we get the following:
* Chamber of Secrets: Length 1.10, Cost 1.11
* Prisoner of Azkaban: Length 1.39, Cost 1.29
* Goblet of Fire: Length 2.48, Cost 2.39
* Order of the Phoenix: Length 3.34, Cost 3.29
* Half-Blood Prince: Length 2.18, Cost 2.39
* Deathly Hallows: Length 2.57, Cost 2.75
Seems like a pretty convincing trend. Conveniently, all were narrated by <PERSON>, so it seems that the costs would be relatively constant (i.e. a similar cost per hour of recording).
Of course, it's impossible to know exactly how the business model for these audiobooks works without inside knowledge, but it's reasonable to speculate that <PERSON>'s payments for each book would be proportional to the length of time needed to record them, so the cost of the product to the consumer would scale accordingly. 862
I would conjecture also that <PERSON> narration contributes to the high cost of the HP audiobooks relative to the novels, considering he's a celebrity rather than an unknown voice actor. Certainly, demand would increase knowing that <PERSON> narrated, and I imagine he is also able to command a greater wage than an unknown voice actor.
RR Voice has an interesting article which goes into some of the many ways that audiobook narrators would charge for their work, such as royalty splits and per finished hour payments (i.e. £X for every hour of recording given to the customer).
Although I can't really say much more without going too far into the realm of speculation, it does appear from the example of <PERSON> that the prices often do correlate with the production cost to a reasonable degree. However, with the many different ways of charging for production, and the difficulty of knowing how much consumers will pay for a certain audiobook, it's difficult to make any more judgements than just observing the link with length. 460