the real question deals with the cost of actually delivering the materials to the Earth. It would probably cost billions to develop the technology and fly a mission to an asteroid, harvest materials, and deliver them to the Earth in a usable form. Divide that cost by the amount of materials each mission can return to get the cost per unit mass for the asteroid materials. If it turns out to be cheaper than the current price for terrestrial metals, then the owners would be forced to find a way to lower their price or face going out of business. However, if the price for the asteroid materials is higher, then it wouldn't be economically viable, at least until the R&D costs are recovered and the price comes down.