ME. Right now. Why would I want to have my tax dollars on this. I have to pay the mortgage. I have to pay the $320 Comcast bill. Going to Mars isn't going to get me anywhere.
It gets your Children, your Grand Children.. or their Children a future secure in the knowledge that their entire civilization might not be wiped out overnight. That doesn't pay your Comcast bill, certainly... but think about why you live, and strive to be alive, life in that sense is simply striving to ensure the continuation of your genes and to provide for the welfare and safety of your children, and your progeny into the future... by that logic alone this is the right thing to do.
More than that, the human race is... to the best of our knowledge at present, the most complex/advanced life in the universe... I think it is our obligation to protect that in the long term, not spreading out into the solar system, and eventually the stars is simply slow suicide for life on earth, and possibly life in the universe.