(Registration required) SpaceX founder Elon Mush says that he can put a human on Mars in 10 to 20 years’ time for $5 billion — or even $2 billion, thanks to the fully reusable rockets he’s determined to build.

Musk wants to put 10,000 people on Mars. “Ultimately … millions.”

He says a reusable version of SpaceX’s newest launch vehicle, the Falcon Heavy, scheduled for testing in early 2013, can deliver a payload of up to 15 tons to Mars at a cost of $100 to $200 per kilogram. That makes his $5 billion humans-to-Mars price tag seem realistic.

Even so, the Falcon Heavy would need to be “heavier” still to carry the minimum 50-ton payload needed for a Mars mission.