He is funding his Mars project from the profits of selling his spaceships to people here on Earth.

I agree he’s probably being over optimistic about the price, unless he has a self-replicating factory skunkworks (which is also entirely plausible), but on the other hand Musk’s estimates are only factor of x2-x3 fantasy compared to subsequent delivered realities, better than the rest of the space sector even when he’s promising more.