Are your friends very experienced? I went to Cal and afterwards went to one of the major tech companies. None of my recently graduated peers anywhere (including Facebook, Dropbox, etc) got anything close to $500k in stock. If you got lucky and timed valuation before a stock boom, you might get $250-300k. In general, the offers were around $60-150k in stock over 4 years. With more normal growth, it could reach $200k. For many people, especially in later years or if you had bad timing, it could be signifanctly less than that.

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.

You are way off. I would estimate $100-200k for 4 years for someone with around 4 years experience when they join. This is based on some quick googling, glassdoor, quora and knowing that 600k+ stock for 4 years at established company for relatively junior positions doesn't make economic sense.

Acquihires and very rare skill holders or more senior roles? Higher -- if your friends of either of these, then maybe.