As far as rare and valuable materials go, I'm not aware of any which are more common in asteroids than they are on Earth. Most space rocks are just the same few elements over and over again; iron, nickel, silicon, magnesium.
"The physical composition of asteroids is varied and in most cases poorly understood" - http://en.wikipedia.org/wiki/Asteroid#Composition
That is just in one asteroid and not a very large one at that. There are thousands of asteroids out there."
Oh, also: the disadvantage of mining the Earth's crust is all those people and other things living on it. The environment is not an issue with an asteroid - you'll be using up the whole thing anyway.
Longer-term, of course, you can set up a big mirror and smelt the whole asteroid in situ for much, much lower prices than a terrestrial smelting furnace - with the added advantage that you'll be able to breathe in Gary, Indiana. (If you still think getting your metal out of the crust of the planet you're living on is a good idea, I invite you to visit Gary. It's way better than it used to be, since China and Korea have volunteered to poison themselves in its place, but it's still bad enough.)