Elon Musk's Mars math (say that 10 times fast) goes like this:
In order for the SpaceX founder's plan to colonize the Red Planet to be economically feasible, the cost of space travel will have to be drastically reduced. So here's what needs to happen. An initial group of 10 colonists will need to pay $500,000 a pop for the one-way trip to Mars. As the colony becomes increasingly self-sustainable -- and fewer resources need to be transported from Earth -- that will free up more space for additional human cargo. Transporting more and more paying customers, and relying on a "rapid and reusable" rocket, is key to Musk's price improvement strategy.
Of course, there are many unknowns about the human capacity to live, and flourish, in space, so the question of how to create a stable population on Mars is enormously complex. For instance, will fertility and mortality rates be similar to what they are on Earth?