I considered the near Earth object case. Clearly that's the easiest place to return material from; the problem is that it's coals-to-Newcastle. So far as we know the bulk of that material is stuff that's easy to get here on Earth: silicates, sulfides, iron, nickel etc. Judging from meteors found here on Earth there are exotic materials like iridium, but in trace quantities.
While there's no doubt lots of valuable stuff like platinum up there, I think people are picturing it as floating around as nuggets of largely native metal. The platinum deposits in Canada's Sudbury Basin were delivered by a meteor, but that meteor was fifteen km across. It contained a lot of Pt in absolute terms, but in relative terms the Pt was rare compared to silicates or nickel. The liquefaction of the meteor in impact separated the heavy metals into convenient deposits. If we tried to mine that object while it was in space we'd have had to crush and melt a lot of ore to get much Pt.