iSuppli analyzed the iPhone 3G now in detail the manufacturing cost after they published a Virtual Teardown of the iPhone 3G back in June. The results are almost the same. Apple has about $174.33 pure manufacturing costs on the iPhone 3G, which is around $50 less than for the original iPhone.
So even while providing more functionality (GPS, 3G) Apple still managed to lower the cost, which enabled them to offer the new low pricing without killing their margins.
As reported, Apple sold 1 million iPhone 3G phones in the first weekend.
More details on Apple Insider.