The survival environment required for humans is several times what is needed for the robots. And you have to ship the humans back(along with a survival environment). So you start to multiply not by 2 but probably start at cost times 10 and up.

Then you compared to having humans building an airport vs robots. But with robots you probably didn't need the airport in the first place. You have to figure out what your ultimate aim is. For the forseeable future when the aim is sheer access of information (prospecting, etc), then infrastructure required for robotic exploration is tremendously less than that for human exploration. Actual colonization and tourism are the only endeavors I can really think of right now that require the human presence. And colonization and tourism will probably be most cost effective if teleoperated robots "set up camp" before humans ever leave earth.

What NASA can't seem to get though their heads right now, is that when planning for human missions 20 years in the future, they should also factor in not just current capabilities of robots, but projected capabilities. If you can't even justify manned exploration now with the current technology, in 20 years the equations will be even more out of whack. It is the beginning of the golden age of intelligent machines. As processing memory and speeds start to approach that of the human brain (in approx 20 yrs), we better start thinking about evolving ourselves or getting out of the way of the machines.

If we don't lower launch costs to orbit by orders of magnitude (e.g. space elevator), I'm afraid all but a few us will remain earthbound. But even if costs remain very high, I do see us planting colonial seeds (with robots doing the heavy lifting) and dreams of humans in space and extrateresterial bodies will inevitably come to pass.