I did not know so many methods have been proposed. The Skylon reusable space plane seems like the most practical and low-cost method discussed here. I think a space elevator would ultimately have the lowest cost per ton, and it is the safest and most elegant solution. But it calls for materials not yet developed.
The initial version of the space elevator would be cheaper than you might think. It would be used to bootstrap larger, heavier versions. If the first one can be deployed entirely by robot, later versions can be used to send people. People would be needed for a really big system, capable of handling thousands of tons of freight per year. Ultimately it would be capable of handling millions of tons. A hundred years from now the terminals might be the largest ports in the solar system, although it is a little difficult to imagine what physical goods people might need to ship around. I suppose most production will be handled locally with universal replication machines. - Jed
