With the iPhones released a few weeks ago, analytical platform IHS Markit has released a new report on the production cost of both the iPhone 8 and iPhone 8 Plus, with that of the 8 Plus being 37% of its retail price.
https://www.notebookcheck.net/Analytic-firm-estimates-component-costs-of-the-iPhone-8-and-8-Plus-to-be-just-over-a-third-of-their-retail-prices.251634.0.html