@Ben Pace: For a brand-new, never-been-tested, fully self-sustaining off-Earth colony, $50B seems super low to me. I think it's got to be in the many many trillions at the very minimum. For some time scales, even infinite money might not be enough. Creating a whole robot/non-atmosphere economy with positive energy ROI is something that no one knows how to do. Much of today's wealth comes from cheap human labor and massive economies of scale. An off-Earth colony would possess neither of these advantages.