The trouble isn't surviving on Mars. The trouble is building infrastructure on Mars that will allow people to survive there without continuous resupply from Earth, at a cost measured in (at a minimum) tens of thousands of dollars per kilogram.
"at a cost measured in (at a minimum) tens of thousands of dollars per kilogram"
[citation (or math) needed]
The minimum cost for sending cargo to Mars even with only current levels of technology is far, far lower than tens of thousands of dollars per kilogram. A reusable Falcon Heavy (which is being researched currently) could potentially launch payload to Mars for circa $100/kg. To put it another way, that's 10,000 tonnes per $1 bil.
That's the expendable Falcon Heavy. SpaceX is also working on a fully reusable Falcon 9, which would naturally allow for a fully reusable Falcon Heavy. The fuel costs for a Falcon Heavy are around half a million dollars, if we posit that overall operational costs per flight could be kept to around 1.5 million dollars then costs of delivering cargo to Mars would be around $100/kg. That's a pretty straightforward extrapolation of current trends. If we imagine some ways to make the process more efficient we could imagine batching up cargo in LEO and pushing it to Mars via electric propulsion (a proven technology), for example.
These are pretty decent conceivable practical minimums for cost of shipping cargo to Mars within the next 20 years, though any estimate (minimum or otherwise) beyond that time frame would be entirely speculative due to the potential for technological advances.
You claim that the figures for Falcon Heavy cost of launching to Mars are "optimistically low", but that is not the case. Firstly, Mars launch is a well understood problem on the scale of launch to GEO. Second, the figures SpaceX are quoting are launch prices which include a substantial profit margin. The actual costs (especially after development costs have been amortized) are lower. And as I pointed out this doesn't represent a proper reasonable estimate of the floor for launch costs to Mars even over the next 20 years, let alone over the next, say, 50.
