Risk-adjusted returns for startup equity aren't even close to accurate ("let's assume that you're one of the lucky few" isn't a good estimation strategy).
With a 4 year vesting schedule, $200k in stock is fairly normal and $400k is not unusual for a senior hire.
All that being said, money is terrible motivation for working at startup, unless you are the founder and CEO. I left said company after 6 months because the team was dissolving and terribly mismanaged. I had several offers, but the one I went with gave the lowest salary. The draw was that everyone I interviewed with extremely sharp and friendly and that the CEO would allow me to work remotely while I sailed around the world after a year and a half.
That said, there's a sufficient sample of such folks that I am very willing to believe that such numbers are entirely reasonable, and not hard to get.
Acquihires and very rare skill holders or more senior roles? Higher -- if your friends of either of these, then maybe.
Where did you get that figure from? That's a bit surprising to me, there's no way you could get that kind of stock grant as an engineer in any of the big companies I've worked at.