That's not always true. It depends on the maturity of the startup's product but I don't believe (and RescueTime supports this) you can really expect to see long term productivity gains from employees when you require them to work well over 40 hour weeks.
That's where the math comes in. I can't think of a single startup, however, that actually pay half the salary of what a person could earn at BigCo... Mostly because most engineers are savvy enough to know what a rotten deal this is (unless the equity position is dramatically bigger).
