, I'm not as articulate (by a long shot) as the science writers who have dealt with your question many times before, but I'll say my $.02. I think there is a huge disconnect on what we get when we look up (space), down (cellular or subatomic), and sideways (can't think of a good analogy here.) Either way, the money is never spent "up there." The money is spent here, usually in American companies with American employees discovering new technologies that are first used by us. Second, a lot, and I really mean a LOT of technologies we take for granted today (transportation, healthcare, communications, etc.) are a DIRECT result of the money we have spent on NASA and other science organizations in the USA.
The problems we have down here need some pretty new solutions, and they come by finding new ways of doing things. That can be climate change, defense, education, etc.
Again, I know, I really know that I'm not articulating myself well. Look up on YouTube some Neil deGrasse Tyson videos and checkout NASA's spinoff site http://www.sti.nasa.gov/tto/
to see how we are solving problems down here by looking up and all around us.