I'm actually researching this right now for my book. I'm happy to share my work...
It all comes down to energy as the unit of measure.
You CAN harvest asteroids for lead, iron, silicon, and other bulk heavy material. In the inner solar system, the rule of thumb is that a typical asteroid is 600 thousand kilometers (0.004 AU) from the next closest asteroid. The belt is approximately 0.5 AU from Earth.
These are rough estimates, because at certain times of the year many asteroids are much closer. Like Cruithne comes within the Earth-Moon L3. Or Bennu gets a few fractions of an AU from the Moon.
- The cost of transporting one ton of bulk raw material at a slow pace of 18 $ {month} \over {AU}$ is 9.5 gigaJoules per ton.
- The cost of transporting one ton of freight up the gravity well from Mars is 12.5 gigaJoules per ton.
- The cost of transporting a ton of freight up the gravity well from Earth is 62.7 gigaJoules per ton.
So, this sets a "tax" on the price of the cheapest items : water, air, fuel, food, construction equipment and materials. Using modern energy prices of \$0.14 per kWh and the average energy cost of 10 gigaJoules (2,600 kWh) per ton, 1000 liters of water (1 ton of water) has a "fee" of \$370 tacked on, not including harvesting costs.
This above assumes perfectly efficient engines. In truth, if you have antimatter energy, you can probably use the number above. If you have fusion energy, multiply the above by about 1,000 (\$370,000 per ton per AU). If fission, another thousand ($370 million per ton per AU). Chemical engines : not possible.
This stacks. Raw materials generally need to move somewhere for processing, then move get moved to the consumer. Sometimes with more than one intermediary step.
If we have a space elevator (not possible with currently known construction materials) the cost drops to 0 joules per ton. A space elevator sends a counterweight down the gravity well. This counterweight's loss in potential energy "pays" the energy cost for the similar-weighted item coming up. There would be some sort of friction and imbalance, but you get the picture. At the higher altitude of the "top" of the space elevator, the load can be at or close to escape velocity.
The following planets in the solar system can not have a space elevator
- Mercury : too hot
- Venus : does not rotate fast enough (243 days per "day")
- Mars : the precession of the moon Phobos interferes with all possible paths.
Therefore, your poorest person in space needs to be able to generate enough value to be worth the cost of upkeep (either \$370, \$370 thousand, or \$370 million per year in 2018 USD, for example, to be able to earn enough to buy a ton of water, which I'm using as the threshold for "poverty")
There are other taxes. If you want to ship things more quickly, the cost goes up. Rockets aren't 100% efficient.
The outer solar system is a little bit harder. It doesn't show in most models of the solar system, but the inner planets are all within 1.5 AU of one another. The first outer planet is close to 30 AU. There is an order-of-magnitude rise in the cost of everything.
Things are also further apart. The typical comet that could be harvested for carbon, hydrogen, oxygen and nitrogen is 1 AU from it's nearest neighbor.
Radiation is an obstacle. The Earth has a wonderful radiation belt that absorbs most of the bad things out there. It would be an essential pre-condition for either good shielding to be developed, or medicine be advanced enough to deal with radiation damage as casually as allergies.
But, other than that, there's no real limiting factor (in my opinion) to people settling wherever they want.