First - let us recognize clearly and simply that any Mars mission proposed for the 21st century would be only exploratory.
Lower costs and it will become reality. You will never do that with a government program. If you had to ditch a government designed 747 every time you flew across the Atlantic, we'd still be crossing on ships. SpaceX has the potential to lower manned launch to a fraction of current costs. This should help increase frequency, which lowers costs further. Other, non man-rated launch platforms, like the Space Gun, can lower the cost of launching fuel and components into orbit to 1/20th of current prices. And, by the way, this isn't pie in the sky. SpaceX is already profitable! They also have the contract for the next generation of Iridium sats.