You may enjoy this video where a guy goes to Shenzen and tries to build an iPhone 6s from parts. He claims it cost $300. Technically he spent a little over $600 because he broke a lot of parts and bought the wrong parts in the process, but if everything went perfectly it would be about $300. I think this was created back when the 6s was the current model.
$300 claim https://www.youtube.com/watch?v=-KucQDXnKws
build video: https://www.youtube.com/watch?v=leFuF-zoVzA