If cost were no object, it is perfectly reasonable to suppose that by the end of the 21st century, prospering self-supporting colonies might exist on Mars.
They will perhaps have modified the atmosphere to make it compatible with terrestrial flora and fauna, and have warmed the planet sufficiently so that subsurface frozen water will be flowing as liquid and falling as rain. The terraforming of the planet will likely involve bioengineered microbes, as agents of change and sustainers of the new Martian biosphere.
But how to pay for it?
With tongue only half in cheek, let me suggest that we take a page from the opening of the American west.
To encourage the building of transcontinental railroads, federal and state governments gave away huge tracts of land. Six square miles of land was typically granted to the railroad companies for every mile of track that was laid. The companies parlayed free land into big profits.
In the years 1850 to 1871, the federal government passed out more than 130 million acres, or more than the combined areas of New York, Pennsylvania, Ohio and Indiana. Before the coming of the railroads, the government could hardly give away the western lands. In the wake of the railroads, federal land became immensely valuable.
So here's the deal. By a treaty involving all spacefaring nations of Earth, ownership of Mars will be claimed in the name of humanity, then sold to finance exploration and colonization of the red planet. To be sure, this would be a very long term investment, but if the price were right, irresistible.
Land thought to have substantial subsurface water or mineral resources will be auctioned to the highest bidders, most likely multinational investment consortiums. The surface of Mars is roughly 36 billion acres, approximately the same as the land area of Earth. If, say, a tenth of that were sold at an average of $10 per acre, a Martian exploration program could be well under way.
Broad areas of lowlands will be reserved for future seas. And large tracts of land will be held in trust for future public parks, including such natural wonders as the Olympus Mons and Tharsis Montes volcano complex and the Coprates Chasma canyonlands. Also historic sites such as the landing places of the early Martian probes. As the adventure proceeds, more land will be offered for sale. As the first colonies are established -- say by the year 2050 -- property values will appreciate, especially near settlements. By the end of the 21st century, Martian colonies should be economically independent of the home planet.
And what about small investors, like you and me, the sort of folks who bought sight-unseen tracts of Florida swampland early in the last century? I have my Martian plot picked out. I'll tell you where tomorrow.