Congress killed Bush's plan, at least partly because NASA estimated it would cost about $500 billion over 30 years. Bush's son, President George W. Bush, and his successor, Barack Obama, have since held up human exploration of Mars as a goal for NASA, but the funding necessary to make it happen hasn't followed during either administration. [emphasis added]
Here's the problem: they are all trying to do this with BIG ROCKETS. Big rockets are what Congress and big contractors LOVE TO DEATH. But big rockets are killing any missions to Mars because BIG ROCKETS have BIG PRICE TAGS- that's why big contractors love them so much. So, instead of trying to find a way to Mars without these big rockets gets no respect from politicians because those that advocate this don't have DEEP POCKETS. Therefore, Mars has to be done with big rockets, and for that reason, it won't ever get done.
It ain't the lack of funding. It is the insistence upon that which cannot be.