Comparing cost to the employer against the median income is nonsensical. At least try and estimate "median cost to an employer".
Do you think there could possibly be enough slack to justify calling this arrangement "slavery", which was the crux of my point?
I assume your 10% figure here is purely rhetorical, since per my estimate and sibling comment's the cost of employing a graduate student is at least $50k. So your 10% estimate would imply the median cost of an employee is $500k, which is obviously not true.
I don't want to drive this further into nitpicking so I will conclude this exchange by conceding, again, that median income and median cost to employer are not the same thing. Have a good day.