Rand could not have seriously meant that I was ignoring asteroids.
So here I will make the comparison (between a planet like mars vs. asteroids as a mining source) that a comment in a post simply would not do justice to.
Consider your location to be fixed. Where ever you are, there you is. Even so in space (and I'm not being obtuse about orbits, so please.) A mine on mars is a fixed location away from you and can be mined for years. Other minerals, in other mines are also fixed positions relative to you. Even more important economically is you are the master of your own fate. You can individually exploit these resources.
Not so in space. Everything is in motion relative to each other. You live in a can of people, where chances are, you're not the captain. But let's make you the captain because that's an unavoidable fantasy for some. How do you exploit the asteroids? Your choices are... 1) move your can to the asteroid, 2a) move the asteroid to your can, 2b) move some of it to you.
Either option has a delta V cost. No asteroid is going to have the diversity of minerals you need, even if you don't deplete the minerals it does have. So you're back to making that choice again, captain. Your best hope is an ecosystem where other cans of people trade with you... meaning more delta V costs. Which is no where near as economical as hopping into your mars truck and picking up a load from the mine. Planet industry wins.
Well that was a lot shorter than I thought it would have to be?
Other factors? Getting colonist into space is basically the same cost other than landing. But with cans of people, more people means more cans. For planets you can reuse the ships, even landers when they're SSTO, used to transport colonists to a planet. So initially, because reuse takes some development, asteroids win; but soon thereafter planets win again.
The Emdrive may be the game changer.