A personal prediction: I think that Elon will make a round trip to the ISS in a Crew Dragon by 2020. We know he wants to go to Mars. But that is a ways off. In the meantime, once the Crew Dragon is regularly taking astronauts to the ISS, why not make a first trip into space on a SpaceX vehicle, stay a few weeks, and then come back home? He can afford to pay for the trip and it will give him some space experience before making the huge commitment that journeying to Mars will require.