A very similar calculation is why I left a startup recently and went into contracting. In many cases the realistic best case scenario from working at a startup isn't as good (financially speaking) as the non-startup alternatives. This leads many people to extremely optimistic assessments of what the future worth of the startup may be (worth > $1billion)

There are many good reasons to join a startup but if the financial motivation is the main one then it's probably not the best route.