There is more to consider than just page length. To produce an ebook you still have advertising and editing, and most of the other costs of a book other than the paper and printing. So an ebook half the price of a paperback might not be such an illogical cost. On the other hand I would almost always buy the ebook rather than the paperback. First, because they are almost always at least somewhat less expensive. Second, I find physical books a nuisance to carry around and store. And Third, I love having my latest novel on my phone to listen to or read whenever I have the chance.