Manufacturing is $8??? Unbelievable. It's a joke when compared to the rest.
The manufacturing cost is actually higher than it used to be.
Interestingly, Foxconn had a 2.7% profit margin back in 2007, but that has dropped to 1.5% over the years, so they're making less in profit now.
Probably a combination of higher wages, more complicated designs, and Apple negotiating the lowest price possible.
How much more would it cost to build in the US?
Depends on the automation.
IIRC, there was a video talking about an iPhone going through 50 workers to be built, a step at a time.
In the US, most of those steps would be done by robot. And, in fact, that's why Foxconn is buying one million robots over the next few years.
Also, in the US, each soldered board would be examined by automated 3D X-Ray for flaws. At Foxconn, it's only one board out of every batch of ... hundreds? So the US longterm yield would be better.
Apple once made a big deal about how easy it was to get Chinese manufacturing engineers, and about how "all the factory engineering firms in the US would fit" in the exhibit room or something. Of course, that's actually over 5,000 such firms, each with up to a dozen employees. Typical Apple handwaving to avoid questions.
The problem is profit margin more than anything. You could increase the cost of manufacturing from $8 to $80 and Apple would STILL profit more than most any other phone maker in the world. But that's not their style.