This week a report from analyst iSupply said that the new iPhone 3G costs Apple only $173 to make compared to $265 for the original iPhone.

The cost for the iPhone 3G is based on a "virtual teardown" as iSupply did not have a new iPhone 3G available to open up.
The $173 also includes only hardware cost and not the software development for instance.
Apple is expected to get about $500 for an iPhone 3G from carriers (price plus subsidy). If the manufacturing cost is correct the iPhone 3G would be Apple's most profitable device.

Somehow Wallstreet did not really get off on this report yet. The Apple stock is trading way lower than it has early June, when the iPhone 3G got announced at the WWDC 08.

The hype for the iPhone 3G is definitely here. It looks like Europe will take a leading role and by paying with Euro will give Apple even higher margins.
Apple has scheduled its Q3 2008 earnings call on July 21st. The report is supposed to contain first sales data for the iPhone 3G.
More about the iSupply iPhone 3G analysis can be found in this Reuters story.