NYTimes estimates $750 million as possible
See http://www.nytimes.com/aponline/science/AP-Moon-Mars-Cost.html
Unclear where they got the number, except maybe to take a previous estimate for Bush I's idea of direct to Mars without moon landings ($500 billion), and add double the cost of Apollo-era programs to reflect the increased cost of building a moon base versus a flag-and-footprints mission. Times says the inflation-adjusted cost of the moon landings is $125 billion, not 70.
The fact that the Bushies are not floating a number is telling - any number is likely to be unrealistically low or prohibitively expensive.