It may be true for expensive physical products, but when the price is low (as $10 is compared to a $500 tablet), the price is dominated by other factors.
For years publishers had been crying that the reason for the high price of books was due to the high expense of production & warehousing. Enter ebooks and those expenses are gone. Sure there are some new expenses, but nothing even remotely approaching production and warehousing. Logically, the price of ebooks should be reduced but instead have been artificially propped up with the agency model.
"other factors" is just a nice way of saying greed.