It is the nature of technology to be very expensive at first, from cars to computers. The price of 3D printing is bound to come down. The days for printing a phone is still pretty far out though; current printers can only handle a single type of material, usually a polymer.
Additionally, I doubt 3D printers will attain the kind of spatial resolution required to print individual transistors (basic building block of any modern computer) within the next 100 years, let alone in time for the iPhone 8 (even after giving allowance for the 6s & 7s :-). It is, however possible to print an iPhone case today. http://howto.cnet.com/8301-113...