I'd also like to add that the bill isn't "hefty". To one person or cause, it may seem like a lot, but when you see where that money is going then it's no longer just another bill. The work that Nasa does is incredibly time consuming and difficult and it's all done by thousands and thousands of workers. The success of their work affects all of mankind. They don't just absorb money and then occasionally send a flimsy probe into space, it's complex work which receives very little praise in this day and age.
It's easy to fall-back on the age-old argument that there are more important matters that are closer to home, such as disease and poverty, etc..., but Nasa's work is for the benefit of mankind, not just for fun or for bragging rights. Simply throwing money at problems that are close to home is useless in the long run, whereas new technology and information is beneficial for longer periods.
As I stated above, we can thank Nasa for artificial limbs, improved baby food, better LED's, heart pump's for transplant patients, anti-freezing aircraft wings, grooves on roads for better grip, tires that are 5x as strong as they used to be, pH detectors, fire-resistant coatings (used by Apollo), lighter breathing equipment and heat-resistant gear for fire fighters, temper foam, the cordless vacuum and freeze-drying. That's ignoring the breakthrough's for the environment, agriculture, computers, and industrial productivity that were included in that link too.
There's also the small matter of taking mankind further (although that's not reeaaally very important at all, is it?) than it has ever gone before and improving our scientific knowledge with various tests and other discoveries . Oh, and not forgetting the billions of people that US space exploration has inspired over the past 50 years and the thousands of jobs that have come with it.
Take Apollo 8, for example. Had William Anders not taken that photo of Earth ('Earthrise', the very first picture of our planet), none of the 1000+ environmental acts that it gave-way to would have been realised. And we'd also have one less beautiful image of our home as well!
So when you understand what the money is used for at Nasa, it becomes clear that what they need isn't very much at all (especially when you see how much they spend on wars, etc...). But money shouldn't come into taking mankind one step further.
Edit: Could I also add that I spent $48 + entry at the Kennedy Space Center, just over three weeks ago. Some of that money goes to Nasa's programmes, so I actually did help them (even if it is just one drop in a vast ocean that's required to send their astronauts back into space).
