"Most of the present discussion will focus on the U.S. government civil space program. I will have some comments on the international scene and on the possible role of commercial space, but for much of the next five decades, the U.S. government will be the dominant entity in determining the course of human space exploration. We will, I hope, develop robust international partnerships that will enormously enhance the value of space exploration. And we must do everything possible to provide an accepting environment for commercial space entities, standing down government capability in favor of commercial suppliers whenever it becomes possible to do so. But with that said, the U.S. today is spending more than twice as much on civil space, per capita, as any other nation, and I believe this situation is unlikely to change significantly for some time. Commercial space firms offer great promise but, so far, limited performance. For a while yet, it is the U.S. government, through NASA, that determines the main course of human spaceflight."

[I don't mind this conservative approach- esp from NASA administer.]

"Since no aspect of government civil spaceflight is more crucial than the funding allocated to it, let’s consider NASA’s funding history for the last 50 years, and try to make a reasonable yet conservative projection as to what we might receive in the next 50. "

[To get a baseline for prediction this is reasonable.]

"By the 2020’s we will be well positioned to begin the Mars effort in earnest. The lunar campaign will have stabilized; a human-tended outpost will be well established; we will have extensive long-duration space experience in both zero- and low-gravity conditions, and it will be time to bundle these lessons and move on to Mars – which does not imply that we will bring lunar activities to an end. Quite the contrary; my prediction is that the Moon will prove to be far more interesting, and far more relevant to human affairs, than many today are prepared to believe. But by the early 2020s, it will be time to assign a stable level of support for lunar activities, and set out for Mars. "

...
Allocating an across-the-board 30% reserve at this stage puts the cost of a 30-year Mars exploration program at $156 billion in Fiscal 2000 dollars. Of this, approximately $70 billion consists of development cost, with reserve. If $4.8 billion/year is available in the human spaceflight account, then the Mars mission development cycle will require about 15 years. Thus, if we begin development work in 2021, we will be able to touch down on the Martian surface in about 2037, with follow-on missions every 26 months thereafter for the next two decades. "

Important part of article:
"A long-term government sponsored space exploration program carries with it the implicit demand for many tons of cargo logistics and crew transport, offering a stable and tempting market niche for industry. Some enterprises will be surely successful in their attempts to service this market, and from there commercial space activity will bloom. In addition to transportation, space exploration implies the need for communications, navigation, power systems, and other support infrastructure. These requirements will be targeted by specific firms as services to be provided commercially, rather than by government."
And lastly:
"The most important factor for future success is stability in purpose, strategy, requirements, and funding. Apollo funding was unstable in both directions. The huge rate of early growth allowed the Apollo goal to be met; the abrupt cessation of funding as the goal drew within sight produced strategic damage that remains unto the present day."
http://aviationweek.typepad.com/space/2007/03/human_space_exp.html

My comment. NASA is somewhat like the UN. And like UN, if we got rid of it, we would simply need to re-make something like it. This vision is painful slow, but this direction allows the possibility that it could be sped up due to factors outside of NASA. But also NASA the bureaucracy is inherently a slow creature and if you give a stable long term direction it will become FAR more efficient, and it as a bureaucracy could even increase it's pace. I think by generally following this direction NASA could end up "saving" more than 100 billion dollars of tax payer dollars and end up actually getting to Mars faster, safer, and not simply be a "flags and footprints" mission which goes nowhere.
And of course if Congress somehow has too much money, they can give an extra several tens of billions of dollars to NASA and speed up it a bit.
And finally unless you want to scrape NASA this "general NASA direction" essentially costs no extra money to go to the Moon and Mars- individually in total cost, Manned Moon and then Mars will cost about the same as ISS costed. Or Manned Mars will cost less than entire Shuttle program and the Manned Moon will cost less than entire ISS program- and over similar time frames.