iSuppli has done its usual thing with the16GB iPhone 4's components, calculating the raw cost of each of them, and the total comes to: US$187.51. The most expensive thing in the phone is the retina display screen, which is supposedly running Apple $28.50 per unit. The A4 processor adds $10.75 to the unit price, and the gyroscope adds $2.60, as compared to the accelerometer's estimated 65 cent cost.
You may remember that the iPhone 3GS was estimated to have components costing $179 last year at release -- the price on those parts has dropped to $134, even though the phone itself is being sold at $99
The 3GS was also $199 at launch [presumably $599 unsubsidized], which means that Apple is making $10 less per phone with the iPhone 4, but maybe they're making up the costs elsewhere, maybe in advertising or R&D. Of course, these are all estimates -- there are all sorts of other factors going into this cost equation, both internally at Apple and in the manufacturing industry. But iSuppli has been doing this for a while now, and if they say the screen is the priciest component in your new iPhone, they're probably right.