Foxconn puts the labour cost of assembling, quality assurance, and testing of the phone at $15-$30 with a listed pay of $2.88/hour. The 15-30 is based on the 2% to 5% of the phone's costs being labour.

Americans taking the same 5-10 hours might add another $120-$240 to the cost of the phone if they're being paid well for assembly line work. Assuming Apple wants the same profit per unit. If they'll take less, then that amount drops.

The YouTube video they got the 400 hours of work from is from an old article that contradicts Foxconn's statements and job postings. The 400 hours factors in the metallurgy, chip fabrication, shipping, etc. without accounting for the 390-395 hours being spend hauling many chips, fabricating batches, and shit like that.