Then it won't be done.
I'm thinking that's why we've gone nowhere since the Apollo missions, despite our much greater technical knowledge.
And besides Earth imaging, communication/navigation, and the nascent space tourism industry there's not much going on.
OTOH, the "space dividend" is very real. The semi-conductor/integrated circuit was invented by Fairchild to make the weight/size requirements for the LEM and C/SM's navigation computer, which once an orbit about the moon would be out of sight from Earth, and unable to get ground-based computerized course info.
You can argue pretty easily someone probably would have come up with the IC anyway, but it DID actually come from the "space race".
So you might be able to come up with convincing "make work" resources or production on other planets, asteroids, or space habitats. "We'll go, and new industries will arise" etc. Even if it's a false economy, if a self sustaining population on Mars does take hold, it'll be worth it for the survival aspects alone.
And also, heavy automation built from local resources might look completely different in a post-scarcity economy. People will be able to do things just on a whim.
The same could have been said about the America's. Setting up an expedition was ridiculously expensive, let alone keeping the colony afloat.
Sure, it was cheaper and easier to just walk out into the English country side and chop down a tree or get some coal. Only problem was they'd stripped much of their own national resources to the point that it was cheaper to sail it all the way back from America. Same thing with Spanish gold, rum, sugar, and todays doodads shipped all the way from China.
A point is reached where easily accessible resources far away become more economically viable then dwindling resources at home. Once the right combination of propulsion to get the colonists there and the materials home, and large enough deposits of a material here at home that is dwindling, it will be economically viable. The propulsion system probably isn't more then a few decades away, there only needs to be the right resources found that we need here.
There were very real economic incentives to exploit and colonize the "New World". Originally with Columbus, the premise was false, but they had the desire to open a westward trade route to Asia for silk, spices & what-have-you without having to put up with the Middle East. With the timber, you're correct much of Europe was de-forested by then, but the difference in scale between a wind-powered Atlantic voyage for timber, and mining on Mars... is well, economically speaking, you'd be better off planting trees in England and waiting.
After that it was Gold for the Spanish, Fur for the French, and Virgin timber for European shipbuilders.
And even with several orders of magnitude of efficiency in propulsion technology, metals from Mars will not compete with any from Earth. It'll be cheaper to do ocean bottom mining of manganese nodules, direct ocean filtration for metals, and mining our own landfills.
If you try to scale the early European exploration missions and colonizations in the Americas to a Mars mission that will settle a dozen people there or whatever, it's kind of like building a 100 mile long wooden ship, and putting the King's palace and 1000 servants/sailors on it. There's a ton of economic value and force multipliers in our modern technical/industrial society, and it's still a huge undertaking.
And if you get into real Malthusian problems with the population on Earth like you're building a geostationary habitat ring, or ginormous Arcologies/mega-cities etc. Then Asteroid Lunar/mining is probably still more economical than Martian. For an asteroid mine, a robotic mining probe can land, assemble a solar-powered mass driver, and then shovel some of the asteroid's own mass into the driver, and propel it to an accessible Earth orbit. Then you can use large Mylar parabolic mirrors to smelt ore at the focus, and then use mass-drivers again to drop the metals to earth, pre-formed into re-entry aerobraking shapes for splashdown into the ocean for recovery.
In space economy, planets are the low-rent district with high transport costs. The Moon and Asteroids, and the moons of Jupiter and Saturn are the primo industrial real-estate.
http://xkcd.com/681/
