In the 1970s, Princeton physicist Gerard O'Neill led two Stanford/NASA Ames Research Center summer studies that supported the feasibility of kilometer-scale orbital cities. These studies assumed that the NASA space shuttle would operate as expected, a flight every week or two, $500/lb. to orbit, and one failure per 100,000 flights. The studies also assumed that a more efficient follow-on heavy-lift launcher would be developed.
Now the SpaceX BFR being developed over the next 5 years or so could deliver low-cost launch that did not happen with the Space Shuttle. The SpaceX Falcon Heavy with reusable first stages could achieve the $500 per pound to orbit target. A fully reusable SpaceX BFR should be able to get well below $500 per pound. SpaceX BFR has a target of $5 to 10 million per launch of 150 tons. A cost of $30 million per launch would enable $100 per pound. SpaceX BFR plans more redundant engines to improve the safety and reliabilty.