It's not as easy as that. Land is cheaper in China, so the costs to acquire the land for the factory needs to be accounted for.
Also, you need to account for the overhead costs of the factories. Insurance, power, taxes, pollution clean up, repairs and maintenance, garbage collection, etc etc. In each case the costs will be less (and perhaps considerably less!) in China than in the US. I know that Apple is not paying those costs directly (assuming that they would use the same model in the US as China - purchasing from a supplier instead of owning and managing the factories directly).
Most of the parts that being assembled into the iPods are coming from Asian sources. There is a slight cost savings, I would think, shipping a single assembled product across the Pacific, rather than the dozen or more individual parts. In the case of a single product, there is one customs form to fill out. In the case of a dozen parts, each part needs to be cleared - so a dozen or more clearances. And yes I know, they clear entire shipments at a time - not box by box. But it's the difference between a single plane-load of iPods, vs a dozen shipping containers.
Every time you add a little paperwork (shipping, customs, etc) by the middle-people, you add more than a little cost. Each time someone has an expense, they pass that on with their mark-up.
So, looking at the labour costs to assemble the iPods is just too simplistic. You have to look at the whole thing.