1) Economy of scale: For example: Cassini-Huygens is equipped for 27 diverse science investigations. The Cassini orbiter has 12 instruments and the Huygens probe has six. A similar instrument suit for the surface of MArs would cost a similar amount despite having humans on board. There is no increase in complexity there. Orbital mechanics are the same whether you are sending humans or robots. Atmospheric entry is an added complexity, but not one we are unfamiliar with. It also seems to me a pilot would be more reliable than a preprogramed robotic descent. I wouldn't guess you would pay a whole lot more for this than you would a robotic program and it consumed a lot of manpower for Spirit.

At the most basic level it does not cost a whole lot more to design something big to perform the same function as something small when design is a large part of the expense. Using more materials does not inflate the cost linearly.

2) Rule of thumb scale factors: A simple linear mass scale up from the 6 ton Cassini to a 45 ton Mars Direct Hab would give a cost of 21 billion. Given the nonlinearity of economy of scale that should leave a lot of money for human specific work. Off course the Mars Direct plan would involve 2 fuel return modules and 1 hab, but much of the work would be duplicated. This also doesn't account for the development of the launcher so I'm very curious what you estimate for developing Magnum would be given all the work that has already been done on it.

I'm doubtful 20 billion is enough Larry, but I think your claims its not even in the ballpark sound ridiculous. One thing I'm sure of we don't need to spend 400 to 500 billion to get to Mars or build a base on the Moon to do it and far too many people are claiming we do. A Lunar base should be justified on it's own merits not as a way to get to Mars. A lunar base will cost more than the ISS and be of less value IMO. It seems to me even a farside radio observatory (the most unique lunar potential IMO) would have a tough time competing with massive radio interferometers on Earth, much as ground scopes are fast eclipsing hubble even at far less cost. I'm sure you are more equiped to evaluate that than me though.

My view on the Moon is that NASA should evaluate the ice content at the poles, evaluate resources, and build infrastructure capable of returning humans to the Moon (as part of a Mars program). Anything beyond that should be private development unless we see find a science interest as compelling as Mars currently seems to be.

Regards,
Mark