For me the format prices aren't really linked. I've got prices I'm willing to pay for physical, and prices I'm willing to pay for digital. Each format has advantages and disadvantages and I weigh those when deciding what I want to get. I'm the same way with games.
I like to read physical books much more, and I'm willing to pay more for them when I want one. Unfortunately I'm pretty mobile lately (averaging 1 move / year ), which is why I'm even using eBooks.