I don't. I assume it's economics in the sense that consumers are probably inclined to pay less for shorter books, there are a lot of largely fixed costs associated with publishing a book, may even be some pedestrian things like reading a title on the spine in a bookstore. Basically you need some starting hardcover or trade paperback price and work back from there. Amazon has doubtless changed some of the economics especially for ebooks. But, historically, you needed a hardcover you could sell for $25.
I've published a couple of fairly short books I didn't really care about charging for and they just wouldn't have been worth it for any publisher to feed into their funnel.
