Time is your greatest asset, personally and professionally. It's how you stay abreast of the great opportunities of our time. You need that freedom to explore things that pique your interest or move you. Founding a startup means becoming mostly blind to the other great opportunities, which is why I'm so reluctant to do a startup, unless I think it's "the one."
IMO, the opportunity cost of startup jobs is far more sinister than you think. Only do it if you need the money or if you believe you're changing the world in the greatest way you can.