@GeeKay the alleged values don't mean much and any mining attempts, if ever, are likely to start small and probably continue to have a lot of very high costs to recover before achieving profitability - and not much affect the market price.
I am amongst the more pessimistic commenters when it comes to space but the resources in asteroids are real - notably Platinum Group Metals mixed in nickel-iron at 10's of parts per million (going by meteorite samples). Even the nickel-iron, raw and unrefined would be considered valuable here on Earth, just for the nickel content. Most things in space have no potential to make money but asteroid minerals are a real "prize" of enormous potential monetary value, so I think the interest will always be there.
Yes, the differences in velocities are huge; most of any asteroid mining/refining operation would be making enough fuel/reaction mass for the rockets, which must have exceptional durability and long working life. That is probably the first test that needs to be passed - a rocket that can do a round trip between asteroid and LEO exclusively with "fuel" (and other consumables) produced out of asteroid resources. And do it over and over reliably with absolute minimum of ongoing supply from Earth.
Probably not an M-type. We need to know what those rockets will run on and know if a target asteroid has it. Off the top of my head I would target C-type; going by carbonaceous meteorites they contain the target mineral - nickel-iron with PGM's mixed in - as nodules and grains within a softer carbonaceous material that also contains significant amounts of water.
Can solar electric arc-jets use simple water for reaction mass? H2 + O2 chemical rockets present serious problems, including very large tanks as well as, ultimately, inadequate performance. I think things get harder if any rocket uses requires more exotic fuels, eg the Keck Institute of Space Studies proposal to capture a small asteroid and return in to near Earth space with a solar electric rocket using (if I recall) xenon for reaction mass. Seems very unlikely to find a source of xenon in an asteroid. Hydrazine is used with arc-jets and it seems possible (with water and a source of Nitrogen and equipment) to manufacture it, but water, even if less ideal, presents a simpler challenge to produce and store and use.