If we are looking at it like a business then your math makes sense. But as an individual, it would be cheaper just to buy a new iPhone because of the dead ends and time spent assembling the device. So yes if he did everything perfectly the first time then the cost was $300, not including labor or tools. But the actual cost is way above $1k including labor, tools, dead ends and research. A business can factor those costs as R&D as they will producing x amount of these but he is only going to produce 1 and who knows if those tools and parts will work on future projects.