NASA says that the solar system is rich in gold. (And some other things.) Meanwhile, demand for gold on earth has skyrocketed. If the other eight planets (and dwarf planets and meteors) are as rich in gold as the earth is, then there's quite a pretty penny in reaching it. I think the only reason why not is the immense cost of space travel. And there are ways of bringing that down.
A quick investigation into space fountains and railguns could, I think, cut the cost of space travel by a factor of ten. From $10,000/kg to $1,000/kg. Geologists would tell us the most likely locations to find gold, and automated probes would go mine and refine it. The immense cost of setting up this industry would be recouped within a year's operation.
If the price of gold collapses, other materials can be mined as well. Platinum, perhaps, or silver. Even bulk material like iron or lead could be mined in massive scale with no concern of environmental damage, as the non-earth planets of our solar system have no plants or animals to damage. Iron may sell for a mere 2c per pound, but when you can move a billion tons, that adds up to some serious money.
The cost of these materials dropping would be a boon to the manufacturers who make things from them, and and a disaster to the companies who mine it from the earth. (Though, if too many of them object, a change in focus would be doable, maybe even easy.)
Some places would be easier to mine than others. Mars would be easier than, say, pluto, which in turn would be easier than, say, Venus.