What else do they have going on their? It is about the only thing that NASA has project wise right now. But you're right. That would be 0.5 Billion per flight not 5. Sorry bout that. I am not relying on my memory any more!!
Still, that's a whole lot more than the 10.5 million they predicted it would cost to launch. Also, if you take into account the $14 billion dollar overrun for the project development, the space shuttle is one very expensive reusable (hah) vehicle.
ISS
Shuttle
CEV
Unmanned programs, a la Voyager, The Mars missions on-planet and off, etc.
Aeronautics
Life science research
Shuttle and ISS are the two big bruisers in the budget, but NASA is busy with a lot of things.