[Someguywhatever]

I suppose the exception could be rare earth metals, asteroids which contain them in sufficient quantities might be worth going after.

[evgen]

'Rare earth' metals are not actually rare. The cost (both capital costs and environmental costs) to refine them are high, but there is plenty of ore available if you have the refining capacity. There are some elements that are actually rare (e.g. tantalum) but there is no reason to believe they will be any easier to find in asteroids and the cost to refine will be even greater.