I feel that where the factory is located doesn't make too much of a difference. It must require much fewer employees to operate a string winding factory than a guitar factory, for example. All the extra cost of giving American's a livable wage also probably doesn't have as much an effect on such a cheap product. I'd expect all American companies to do it.
I don't know the exact costs, but if it costs $.50 to produce a set a strings in Mexico and the math works out for the strings to be most profitable at $4.50 or it would cost $.65 to produce them at an American factory and you could sell them for $5.25, it would be better to make them at an American factory.
What D'Addario is doing is an excellent idea because it allows him to manufacture in-house from start to finish, keeping costs to him at a minimum.