The price would collapse from excessive supply before the mining costs could be recouped.

Though there are companies seriously considering mining asteroids, they're supposedly cheaper than the Moon because they have no appreciable gravity.

Mining asteroids will be crucial for space economy, but there's little point in shipping mined resources downwell. What makes sense is to move manufacturing of space-related hardware and possibly of things benefiting from fabrication in microgravity up there, and then use asteroids and the Moon as resource base for building things in space, and mostly for space.

No. Even if it were pure platinum that required no refinement it would still be more expensive to return it to earth than to mine and refine earth platinum.