There is one place cheaper
Literally a few million American prison inmates, a large percentage of whom are "employed" as Prison Industry workers. Most are making office furniture for government contractors. Just one of the perks of the increasingly privatized US prison industry.
But then again, it would be a lousy public relations hit for Apple. Nobody cares about offshore economic slavery, but when it's Uncle Fester in a California prison ... that hits a little too close to home.