1) Vast majority of time of manned missions is spent on survival rather than science (see ISS). Therefore effective productivity is much lower than a "walk in an Earth desert."

2) Robotics keeps improving and the effectiveness per launch weight keeps improving. In 2 decades, the key insturments and electronics may well be a 100 times better than currently, so comparisons to robotic productivity should not be compared to current capability but to future capability of when a manned mission might be possible.

3) Humans will only land on one Mars site and will be limited in mobility either to their legs or to...a manned VEHICLE.
Robots mobility could be much greater and cover much greater areas by landing in multiple sites and using balloons, airplanes, and rover vehicles that have large ranges. Those rovers don't have to carry manned survival equipment, nor return to a base like they would if they carried humans.

4) Robotic improvements will not only help in exploring Mars more rapidly, but all the outer planets, asteroids, comets, etc. Does a human mission to mars lead to another 50-100 Billion mission to somewhere else? I don't think so.

5) A manned mission will still require tremendous robotic support, so in all likelihood you have manned costs on top of robotic costs. You will need to send robots to set up a base, to ensure fuel and water, to keep on orbit operations, etc.

6) There are many unknowns to a manned mission, and huge risk of having all eggs in one basket. For the cost of one manned mission, you could have dozens of robotic missions and a large robotic infrastructure set up to do truly planetary exploration vs 1 potential site exploration.

7) I would posit even if you did a manned mission, the manned mission would be much more effective scientifically to teleoperate robots from orbit rather than land on the surface.

8) The technology to send the payload of humans plus all their survival supplies and support equipment back from Mars could alternatively be used to send large numbers of samples from multiple sites on mars. The detailed study of these samples on earth with the vast resources and constantly improving instruments would far exceed any in situ study on Mars. This nullifies much of the reason to do the human exploration.

9) I desire manned exploration and colonization of space and other planetary bodies as much as anyone. I just don't believe it is practical as compared to robotic exploration, and won't become practical until we have some major breakthrough in getting payloads to space. It will take something like the space elevator (which I believe has a good chance of becoming real) to make manned exploration and colonization practical. Putting large number of dollars into technologies that don't meet requirements for sustained exploration doesn't speed human exploration, but slows it. By consuming precious resource dollars for the wrong technology, you only slow the development of the future technologies that will make the exploration practical (e.g. space elevator).