The value added in the final assembly is relatively small. Most of the cost is in the factories in Japan and Taiwan where the components are actually made, in highly automated factories. Screwing the pre-made pieces together is a low-level task.
The actual salaries in China are between $1.70 and $2.50 an hour, but lots of overtime is available, and the cost of living in China is much lower than in the US.
Those are average salaries for most workers. Apple contracts specify even better pay. . . those jobs runs to $3.40 per hour. It's one of the reasons that when openings occur for Apple assembly line jobs that thousands of workers queue up for the openings, even though they may already be working at he same plant. It's the equivalent in China's economy of earning $24 an hour in buying power for most commodities since one can rent an apartment in the city for about $70 - $100 a month. Dormitory space with board at the factory is $50 a month. Typical workers can work up to 60 hours, adding 20 hours of overtime (they don't pay time and a half in China, unfortunately).
Apple's contracts have been one of the primary sources of upward pressure on factory wages in China for the past ten years. . . and pressure for improved worker conditions. Apple maintains paid monitors in its supply chain to assure compliance with its worker safety and condition policies. These monitors are Apple employees, not employees of the contract companies.
Contractors know that Apple means business on these contracts' worker conditions. They've seen Apple follow through with the contract conditions requirements to both Apple's and the contractor's detriment. Apple has actually pulled a $2 billion contract in mid-contract and gone with a higher bidder when a supplier did not clean up its act after several stern warnings from Apple about worker maltreatment. Pulling that contract even delayed release of an Apple product, and affected the bottom line of Apple for that quarter and year.