I will start by saying that I am anti-Apple, and have been so for decades. I think Apple products are over priced, and under perform.
That said, the cost to manufacture one more of any mass produced product is almost totally unrelated to profit, and is usually almost trivial. The true costs include product development, marketing, customer support, manufacturing tooling, and other capital costs. It's simply not realistic to subtract the cost to make one more unit from the sales price, and call that "profit".
It not just the software, administration etc. etc. You forgot to include the cost of the foam parties Apple hosts when developing new iPhones