Long ago, I described a plan for terraforming Mars, a plan that would vastly increase the territory available to Earthly life, including our own. It would be slow and agonizingly expensive, but worth it. Now, here's a second means of doing it, far more slowly, but in more affordable bites.
We start with a robotic probe, steel pieces, a tank of compressed air, and plexiglass. The probe lands, finds the steel, and assembles it into a frame. It then slides the plexiglass between the steel, and seals it airtight. This structure should also have an airlock. On the inside, leave the air tank, preferably opened. When the pressure is earthly pressure, (760 Torr, or one "atmosphere"), then we have something very valuable indeed: a base of operations on Mars that could sustain a small group of people. The first group to use this should be with them a collection of plants, or at least seeds. This will keep the air breathable by humans. They should also attempt to farm to reduce the amount of supplies needed, and dig an underground complex for when radiation levels come high.
One of the missions of human visitors to Mars should be to build additional complexes. This will allow expansion of the number of missions, and will cost less than the original, robotic one. Some of these may even be sold as exotic living space, which I'm sure a few people can not only afford, but want to buy.
When we have a number of these, we can attempt megaprojects to reactivate the Martian core, resupply the Martian atmosphere, and melt the ice on Mars. A few meteor supplements can fill out the Martian sea, and when an atmosphere is established, then plants can be planted outside to make it human breathable. In the end, Mars would be as friendly to Earthly life, humans included, as the Earth itself.
This project would run for millions of years in total, but at no point would more than $1 billion be needed. Mars would slowly gain value as the project progressed, as it gained industry, agriculture, and sustained more and more people. Ownership is an issue, as a treaty in place forbids national ownership of the turf in question. All space powers, including the United States, have signed this treaty.
I think that within the next thousand years, this slow method or the fast method, will become necessary for the sustaining of our collective economic systems, which demand perpetual growth. This requires an ever-growing population, and an ever-growing amount of factories, mines, and so on, and the Earth is only so big. As it grows, we increasingly have to make some hard choices. We can spend trillions growing Mars, or we could sacrifice wild lands, or we could expensively build under the sea. Any of these options have some painful drawbacks, but we must persue one.