For a Death Star.
Actually, as far as government spending projects go, I've heard worse ideas.
(hat tip to Robert)
Update: In retrospect, this figure is grossly inflated. $12.8 septillion is devoted to actually putting the estimated 134 quadrillion metric tons of steel into space. That's based on current Earth-based space-faring rates, which of course come from incredibly inefficient rockets that can only put a few tons in space at a time.
Any civilization that can figure out how to move that much mass from one star system to another (say, from Alderaan to Yavin) in a matter of hours or days has obviously come up with a much more efficient way of moving objects into space. (Even old beat up land vehicles seem to have no problem defying gravity in this world.) I'd guess the costs of moving the steel into space are pretty negligible. This project wouldn't cost more than a few quintillion dollars.
Also, they could probably come up with something lighter than steel. Just guessing.
Still, isn't the whole Death Star a really wasteful project? I mean, basically, all you really want is a giant gun that can move quickly from one planet to the next. It just has to be a credible threat to disobediant systems. And the gun itself doesn't seem to require too many people to operate and maintain it. Do they really need all that other stuff and personnel in there?