* Google salary is $180k
* Startup salary is $125k
Expected value of stock options must be >$55k per year to be economically equal. If we assume 5 years to exit, 20% odds of successful exit, and $50 million exit, that means I must own about 3% of the company at the time of exit, assuming I have no risk aversion (and more if I do). Assuming reasonable dilution over the 5 years, I need to get substantially more than 3% today.
In most cases, only founders get that level of equity. The equity offered to future employees is, quite frankly, a complete joke for most startups I've seen. The assumption is that there aren't enough developers who can't value options well that some sucker is gonna get fleeced.
That doesn't make startup the wrong option (I went with it), but the incentives have to be non-financial. Startups can be great or horrible places to learn, depending on how much exposure you have to business processes and similar. In Google, you're in a specialized box. In startups, you can see many more aspects of the operation. Getting into a position with that kind of visibility, however, can be difficult to properly negotiate early in the career.
That depends entirely on the company and the developer, there are no absolute numbers. Keep in mind though that often it's entirely fair that even the earliest employees get vastly less than the founders. After all, they probably didn't spend the last three years living in their parents' basement to get the business to the point of being viable for full-time employee #1. E.g. I think it took Kickstarter something like seven years to actually get to launch.
Most likely the equity will be worth nothing or very little - I thus value it at zero so I'm not willing to give up any amount of salary for it.
However, with that in mind, I will still give up salary in return for equity if I am (a) passionate about the business idea, (b) believe that there is a good chance that the business will succeed, (c) believe in the founders and team that is already in place, including any investors. And looking back (I am in my 40s now) the younger you are the more risk you can absorb in terms of equity. I enjoy the startup world and am willing to take risks. Plus, like any other job if it doesn't look like it is panning out or you are unhappy there is nothing holding you there.
Most developers get tiny, tiny amounts of equity at startups which don't pay off in the end.
And the appeal of "interesting work", soon dissipates when your up against deadlines, and the focus is speed not quality code. As is in most startups.
Just working at a successful startup won't automatically bring success, you have to be in a position to capture that success. The easiest way to do that, is be a founder.