Here is VERY brief summary of Zubrin's model to get to Mars. Again, a significant savings is based on using available technology such as the Energia booster...very high payload capability and man-rated. Fascinating concepts....
The plan involves launching an unmanned Earth Return Vehicle (ERV) directly from Earth's surface to Mars using a heavy-lift booster (no bigger than the Saturn V used for the Apollo missions), containing a supply of hydrogen, a chemical plant and a small nuclear reactor.
The ERV would take some eight months to reach Mars. Once there, a relatively simple set of chemical reactions (the Sabatier reaction coupled with electrolysis) would combine a small amount of hydrogen carried by the ERV with the carbon dioxide of the Martian atmosphere to create up to 112 tonnes of methane and oxygen propellants, 96 tonnes of which would be needed to return the ERV to Earth at the end of the mission. This process would take approximately ten months to complete.
Some 26 months after the ERV was originally launched from Earth, a second vehicle, the Mars Habitat Unit, would be launched on a high-energy transfer to Mars carrying a crew of four. This vehicle would take some six months to reach Mars. During the trip, artificial gravity would be generated by tying the spent upper stage of the booster to the Habitat Unit, and setting them both rotating about a common axis.
On reaching Mars, the useless spent upper stage would be jettisoned, with the Habitat Unit aerobraking into Mars orbit before soft-landing in proximity to the ERV.
Once on Mars, the crew would spend 18 months on the surface, carrying out a range of scientific research, aided by a small rover vehicle carried aboard their Habitat Unit, and powered by excess methane produced by the ERV.
To return, they would use the ERV, leaving the habitat for the possible use of subsequent explorers. The propulsion stage of the ERV would be used as a counterweight to generate artificial gravity for the trip back.
The initial cost estimate for Mars Direct was put at $55 billion, to be paid over ten years.