It requires a huge effort to recover materials from levels that low. I wasn't even thinking of trace levels like this because they are so impractical. I was refering to resources like the supposed water at the poles. This does nothing to change the slam dunk difference on resources, it just shows that you could in theory eventually self support a lunar based after you built up a tremendous amount of industrial infrastructure as opposed to using something closer to your pool cleaning apparatus combined with your home appliances. Gas processes are easier than liquid processes which are far far easier than solids processes. Solids based processes are far more prone to failure due to abrasion and cross contamination. Even when it comes to solids processing there's a big difference between recovery from trace quantities and the use of concentrated ores. Every order of magnitude in processing volume is going to increase your costs by at least a factor of 2-3 normally especially when it means more massive equipment launched from Earth.

My basic point of view is that colonizing space is going to be hard and those that claim it's impossible like those that doubted manned flight have some basis for their opinion. If we are going to try and do it we need to follow the easiest path possible. A slight increase in risk for a huge increase in chance of success is worth it IMO, but I think there are many factors where the risk is actually reduced on Mars. For example I doubt Martian dust is as abrasive as lunar dust since it is formed from wind erosion while the Moon's is impact generated (ie glass shards). I also believe the problems associated with low g will be much less if not negligible on Mars. The Moon also provides 0 protection from micrometeorites while the Martian atmosphere provides some (don't remember to what size, certainly much smaller than Earth's). If I recall correctly the rule of thumb is an equivalent mass of atmosphere. A terraformed Mars would be almost as safe as Earth and semi-stable over thousands of years while no realistic potential for such exists on the Moon. Solar flares will be somewhat less of a risk due to attenuation with distance.

Regards,
Mark