Thank you, Glenn, for pointing out the blindingly obvious to people who really ought to know better.
$100 billion just to go back to the Moon? For what? Is there suddenly a critical shortage of moon rocks that threatens to wreck the economy? No, wait -- I know! We've just discovered there's trillions in oil on the moon! And no environmentalists or medieval regimes! A huge cache of diamonds, perhaps? The secret to a grand unification theory of relativity and quantum mechanics?
No, sadly, there is no reason to go back to the Moon, except that it's there. There is, however, a reason we never went back: it's a big, airless, resourceless, useless hunk of rock that costs $100 billion to get to. We have rocks here on Earth. They cost a lot less than $100 billion to get to.
A space elevator, otoh, not only serves a useful function (imagine that, a space program whose primary objective is something that's actually useful), but would be an epochal achievement like the Moon landing of our parents' generation and the Panama Canal of our grandparents' rolled into one. Not only would it be a tremendous achievement in and of itself, it could serve to usher in a new era of low cost orbital transport. Just imagine how the world will change if satellites can be put into orbit more reliably than today for a tenth to a hundredth of the current cost.
Now, I understand why NASA has not yet fully embraced the space elevator concept, though they are looking at it, including sponsoring a competition for the "climber" portion of the elevator. When I introduce the concept to engineers for the first time, they give me a look like I've just claimed Elvis has been living in my basement all this time -- till they see the white papers. Then they scratch their heads and say "Wow, that might actually be do-able." In the end, it still may prove not to be feasible (though that looks less likely every year) but at the very least NASA owes the idea a few billion in development money. It's a lot better than wasting $100 billion on a been-there, done-that boondoggle.
Best case: sometime in the next couple years, NASA says the space elevator looks promising enough that they cancel the whole "Operation Do What We Already Did 50 Years Ago" and divert that entire budget into building a half-dozen to a dozen space elevators.
UPDATE: It occurs to me now that if the space elevator comes in anywhere near its projected budget, the cost of going to the Moon would probably actually be less if we diverted part of its budget to making a space elevator to help us get to the Moon. As Glenn points out, most of the energy cost is getting into orbit.