Currently, all resources off planet are fairly expensive to get at, to say the least. But there are proposals out there that at least seem to bring about an economic case. One such proposal is below.
A note: I disagree with his assumption that no space minerals can be mined economically today. I would say that we already have the technology to do near earth asteroid mining at a cash flow positive rate, given the current high prices of gold, platinum, and other metals.