The problem with mining either the moon or an asteroid is ownership rights. Thanks to international agreements pushed by organizations like the L5 society, it is not currently legal to claim any celestial object. This places mining rights into question. This is probably one of the major factors that has slowed down space exploration: it isn’t profitable and accessing material that is needed may technically be illegal. That is something we will have to work through if we are going to achieve the goal of living in space or on another planet.
Again, I am using very rough guestimates here. Better numbers would require a larger study. However, there is no reason why construction could not be completed within the following timeline.
Construction of a mining system on the moon would be the first step. Assuming we get past the legal complications of mining, which may become a moot point once someone actually starts, considering that it is likely that no government will take legal actions against such a mining institution, with current technology, this could be completed in as little as ten years, or even less.
During this time we could start planning the design and construction of the new space station. The bulk of the space station’s parts could easily come from Earth. This would mean that we could finish the construction of the station at roughly the same time as the mining system goes online.
After that, it would only take a few years to prepare a site for habitation, especially since the site would not have to be entirely self sufficient at first. So overall, it would take between fifteen and twenty years for us to be ready to truly send people to Mars, at which point, it would not have to be a one way mission. Of course, in theory we could send people there sooner and hope that we get our act together so that in by people arrive on Mars in 2030, they would only have to wait another five to ten years before they can return home, if they wish to do so.
Note that, assuming it takes only 15 years to be prepared for a permanent settlement, that would mean that the estimated annual cost of the preparations, assuming it cost anywhere near as much as estimated, would be $147B.