are we talking about assemble in the USA? any low labors cost country can assemble the phone. it doesn't have to be China. Foxconn already have a factory in India and Samsung in Vietnam.
design can be done in the USA like Apple. component like corning gorilla glass is made in the USA. chips can be produce in the USA with Intel IDM 2.0 or TSMC in Taiwan.
it will cost the same since you can outsource assembly
this is your question and I asked if we are talking about assemble the phone in the USA or not. if not assemble in the USA then the cost won't change much.