Could An Economy On Mars Ever Be Viable?

With billionaires like Elon Musk and Jeff Bezos setting their sights on space exploration and development, there must be some commercially viable industries in space and on Mars, right? Well, it's a little more complicated than just mining the asteroids that contain trillions of dollars of value, or building heavy things in micro-gravity, and certainly a base on Mars might cost more than it's worth. Making Mars a viable economy might not even be possible.