That startup's stock is not liquid, and that %0.5 is subject to a large amount dilution on top of it if it ever becomes liquid. For example, it's often not worth it to work for a startup, even if you get that %0.5 equity at the $10mm valuation, since even if it gets to a $1 billion valuation, it doesn't mean you can sell it.
Dilution would then make that $50k into $500k instead of the $5 million you would expect. $500k / 4 = $125k / yr (vesting), which is the same as working at google / apple if you could sell all of your stock. But you cannot so you probably will not get that money back. So a very good case scenario for startups can lead to getting the same money as working at google.
With the current trend of 8+ years to liquidity if the company is successful, it will be a long time before you get your $50k back. Then you have to compare putting that held up $50k in to an index fund over the same 8 years.
As a result, you should probably be demanding something like %5 of the company. Dividing your missing compensation by the probability that you would actually get that missing compensation will lead to a calculation like that.
They will predictably balk at the %5 suggestion, so it's up to you to figure out if you can make more money working for someone else keeping your sales costs in consideration. If not, then go ahead and work with these people, and just treat the equity as the lottery ticket it is.
http://techcrunch.com/2011/10/13/understanding-how-dilution-...