Let us build a iPhone 4s. It will cost like this :
Memory : You need to costly memory chips....one NAND storage and a RAM.A 16 GB NAND flash will cost you just about 19$ and an 512MB DDR2 mobile is 10$ less.

The display unit will cost you 37 USD. You will have to buy a 'Retina' display (960 x 640) and a touch screen.

The brain is an A5 chip (15$), 1Ghz Dual Core ARM.