apple sells it for around what the 5 would have been after the 5S is out, but by putting a plastic back on it they probably keep a similar margin to before. And they can offer different colours for those that care
(hugs red lumia 920)
You should consider all hardware models introduced to be engineered for lower cost to produce over time. The iPhone 5 is a failure of a design if it is unable to reduce adequately in costs over the course of the year. The CNC machines are bought and programmed for, and every component is over 1 year into economies of scale that should have adequately reduced costs (remember that components are also used in iPods, iPads, and Macbooks, so they further increase the scale that brings down the cost of components).
I think the plastic iPhone is not to maintain margins on the high end, but to maintain margins on a lower-end product. They can produce aluminum 4" iPods for $299 with dual cameras, including a 5 MP camera. They can add cellular antennas, increase battery size, and still maintain as high a margin as the iPods (possibly higher) if they substitute plastic for aluminum and sell at $299-$399.