The cost to mine, refine, and transport a product to market is not zero. The overall costs to mine and transport an asteroid’s materials have to be less than the value of the end product to achieve profit and justify the venture.
Materials in the ground are not counted as a reserves, unless there is a established economical extraction process allowing at minimum a return on the investment (ROI).
This is the situation shale oil faced in past episodes when price bbl. dropped due to OPEC manipulation. Extraction costs were eventually reduced enough to allow shale oil production to continue with OPEC oil in the $(50-60) bbl. range.
The production costs vs expected returns is what makes it uneconomical.