Neils degrasse Tyson has a really good video presentation he made at some UAE conference that's on YouTube somewhere. I normally don't listen too much to the science popularizers (Bill Nye, Tyson, Kaku...etc) that get consulted to talk about subjects outside their specialty (Example: talk to a geologist or whatever and not a string theorist when discussing plate tectonics). However, this one got my attention.
NDT says that people do big expensive projects for 3 reasons:
1.) War (people will do anything not to die like build a great wall or an atom bomb), 2.) For religious reasons or to satisfy a monarchy(pyramids, Notre Dame cathedral), or 3.) For economics.
We spent $$$$ on the space program as it was a key part of showing off our might in the cold war (#1). NDT speculates that Mars will not happen unless there is a military or economic reason to go there since #2 isn't big anymore.