Apple will only charge what people are willing to pay. If fewer people buy the new models at $750 or more, they'll cut prices. If they sell all the new phones they make, the next models will be more expensive. Eventually they'll figure out the top price people will pay for a phone.
It has nothing to do with manufacturing costs or anything like that. It's all about the price people are willing to pay. Apple has been just jamming features into the phones to attempt to justify the prices they charge. How many people actually use 3D Touch, Animoji, Slow Mo, Live Photos, etc? I'd imagine most of these features are not used very often by most iPhone purchasers.
This. Not so hard to understand.