On the sapphire element, obviously it is going to more expensive to some degree than an established commodity (reinforced glass). However, I don’t think you can know how much more expensive it will be to produce when Apple is the one doubling the worldwide output of sapphire to use solely for itself.
On the PPI jump, I get the feeling an increase of 100 PPI, while impressive as a technical specification, isn’t going to translate into a meaningful real-world difference. Modern Android phones have displays with PPI in excess of 400 and I can’t tell the difference pixel-density wise between those and the iPhone’s 326 PPI display.
So why go to such an extreme then? I think it is merely a by-product of Apple wanting to go above the resolution of full HD, 1920x1080. What I’m saying is I don’t think Apple is increasing the PPI so they can advertise a better PPI — this characteristic isn’t going to be called ‘Retina+’ or anything.
What Apple can advertise though is the ability to watch TV shows and films at full resolution. If you are making the screen bigger, you have to change the resolution so you might as well kill two birds with one stone. Going to a 5.5 inch screen yields a 400 PPI alone. It’s not that much more of a leap to land at 440.