A competent engineer designing a devkit as simple as an arduino needs about 1 day of work. Give it a week to include debugging. Amortize that cost over a million units and engineering time comes out to less than one cent per board.
It’s not quite that easy, and besides the hard part is the SW. arduino spent years writing SW code and still does to make it easy to run, debug issues and provide support.
Also a million dev kits is unrealistic for vast majority of companies 5-20k is more the number I hard.
> A competent engineer designing a devkit as simple as an arduino needs about 1 day of work.
Hah! I like to underestimate scope as well, but this is really something else. Definitely a competent engineer could make something like this. But a couple of months maybe. You won't even read the documentation for the chip in a day.