I don’t
see the savings unless someone has some sort of
technology that will make mining asteroids much less
expensive than earth metals and minerals.
See, “Cowboys and Aliens”.
Well, the economics just don’t work for mining anything remotely common on earth, for the reasons you point out.
However, for some minerals that are very rare on earth, but common in space, the economics might work out. You would have a very large upfront cost setting up the operation, but the continuing cost of operations wouldn’t need to be that high if you planned it well. As soon as it was up and running you would essentially corner the market on a valuable commodity by controlling the majority of the supply. That advantage would disappear as soon as a competitor shows up though, unless you set up a cartel like OPEC to stop anyone else from flooding the market.