No chance the metals would become "too cheap to meter". I'd bet on the opposite in fact:

step 1) acquire the stockpile of metals

step 2) set prices just low enough you drive all current mines/producers out of business

step 3) quadruple the prices. It'll take people a long time to spin up new mines and refineries. For bonus profits spend some of your monopoly money on lobbying/regulation.

step 4) goto step 2