That's perhaps ~10x less than is typical.
I'm not saying your general intentions are wrong, but you should be able to get a much better deal from startups than you have seen to date if you want. I'm moderately surprised at your experience.
$250M valuations are quite an achievement already and $500k, while it will pay off a mortgage, is not life-changing. (not talking out of experience here :)) So, an early engineer who will most likely be very instrumental to bringing the company to such a valuation ends up with a(n admittedly very) nice bonus, while founders do end up with life-changing wealth.
The categorial difference is that if the company flops in a few months the founders are out a lot of money, whereas as first engineer you are merely in a similar position to before you took the job, but with some nice experience on your resume and a few months worth of pay in your bank account.
[1] http://www.avc.com/a_vc/2011/12/burn-rates-how-much.html
That's the fully loaded cost of the employee, so the sum of salary, payroll taxes, health insurance, unemployment insurance, office rent, and whatever else I'm forgetting.
Edit: also called the fully burdened cost
Most founders I know would pay themselves a salary after the first funding round. A small one perhaps, but I don't think the difference is as big as you make it out to be. Especially when you consider that the first engineer in a startup gets at least the same workload as the founders.
Not at all. An Engineer can only have one job at a time so they most likely quit one to get this one. Now they have nothing, no income at all. Some founders will be in a similar position but many still won't have to get an office job if their startup fails.