I faced the same decision and decided to go with SpaceX. Going from a job I didn't enjoy to one I love was worth the lower salary. I think 'compensate fairly' is exactly what SpaceX did. They were getting great employees for $X. It didn't matter that I was getting paid $1.2X at my last job or that Google was paying $1.3X. I wasn't drafted, I didn't have to take the SpaceX offer. If SpaceX couldn't get good candidates at $X they would make bigger offers. It doesn't make sense for a business to pay more for an employee than it needs to. I don't pay my pool cleaner extra just because I can afford it.
In my mind, Google is $X and SpaceX would be $.76X. Why is passion a trade-off for compensation? Why did you allow yourself to be taken advantage of, and thus drive down the cost of labor? People like you are the reason wages have flatlined over the past 2 decades.
Please don't transfer your sense of victimization on to me. I wasn't forced to do anything and I surely wasn't taken advantage of. I left a good job to come to this one because my career is more to me than just the money.
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This is simple supply and demand. It really isn't that hard. There are more people wanting to work at SpaceX than SpaceX needs, so that drives the price down. It would be as stupid for SpaceX to pay $X for an engineer that will take $.76X as it would for them to pay $100 for a box of pens that Staples sells for $20. Google can get away with it because they are so profitable. Overspending on employees helps them keep employees and attract new candidates. Really, who has a passion to optimize YouTube load times? Google overcompensates to generate passion for working at Google. SpaceX doesn't need to generate that kind of company passion because working in spaceflight is the passion. Not that company passion doesn't exist, it just doesn't require overcompensation.
Forget about companies and employees. Let's say you have a house and want to add a pool. If you think it will cost about $20k and the first quote is for $18k, do you tell the guy you want to pay him extra? Do you tell him he's letting himself be taken advantage of and that he's why the local pool builders can't raise their prices? What if your favorite builder wants $22k? Are you are horrible person if you negotiate him down to $20k?
And since this is Hacker News, let's not forget about all those suckers working for FREE on Open Source. How responsible are they for programmer wages not being higher? How many of us didn't get jobs writing commercial Unix, web servers, browsers, etc.
If YOU don't want to work for less than $X, then that is your decision. Don't apply to SpaceX. They aren't promising one salary and then shorting you on your paycheck. You get an offer and you take it or leave it.
http://tmsearch.uspto.gov/bin/showfield?f=doc&state=4803:1g9...