With the current plans for 2011, ~50% of NASAs budget is for military satellite launches, and so will have little to do with actual science or long term NASA programs. With what is left NASA has to redesign and replace their shuttle fleet (The last shuttle built, The Space Shuttle Endeavour, cost $1.7B just in construction, that doesn't include maintenance and operation costs). And while $18.69B might seem like a lot, it makes up less than 2% of the governments defense spending. And when they have to scrap and/or shelve their programs and re-focus on new goals every 4 years when the government agenda changes, a few billion really doesn't go far.
A lot of people complain about the amount of money spent on the space program because there are few immediately tangible improvements to American life. Everyone seems to forget that without NASA, we wouldn't have the advanced communications networks, offensive/defensive missile technology, weather satellites (or satellites of any kind). Hell even around the home smoke detectors, water purification systems, the padding in motorcycle/sport helmets and safety gear and even Black and Decker invented portable power tools for NASA. Not to mention that the pure sciences researched there have helped thousands of other fields, helping everything from digital image analysis of medical images such as MRI, to digital photography, to radio communications, electronics, computers, aircraft of all kinds,aerodynamics, remote control devices and thousands of other fields.