With its annual budget of $18.724 billion maybe? Seriously, where does that money go?
* Operating their current missions http://www.nasa.gov/missions/current/index.html
* Designing and building future missions http://www.nasa.gov/missions/future/index.html
* Public outreach/education
http://en.wikipedia.org/wiki/Budget_of_NASA
It costs a lot to engineer and operate things that work in space.