I've actually looked at it, taking all $3 billion of Amazon's 2017 profit and dividing it among its 500,000 employees adds up to 6000$.
To be fair though, they aren't making money on fulfillment really, they're cross-subsidizing it with their more profitable segments which also do not require that many workers. In that sense it isn't comparable with Wal-Mart and McDonalds.
I used to work in warehouses, years ago. Even $1000 would have meant an extra trip to the dentist that year and maybe some new clothes more than once per 2 years.
To be fair though, they aren't making money on fulfillment really, they're cross-subsidizing it with their more profitable segments which also do not require that many workers. In that sense it isn't comparable with Wal-Mart and McDonalds.