Problem is that mining on Mars is far more involved in terms of energy consumption. You have pretty much the same challenge as on Earth, needing to dig deep to reach ore. Then transportation of that ore will require a lot of energy.

An asteroid in contrast typically contains far higher concentration of valuable minerals. There is much less to actually mine. And while the transportation of ore takes some time it requires extremely little energy since there is no gravity to contend with.

But just to meet you half way: Setting up the initial mining operations will of course be MUCH MUCH easier on Mars. We can use similar technology to what we use on Earth.

Getting the first asteroid mining going will be difficult and expensive. However my claim is that once this as been mastered, asteroids offer a much better deal. Minimal costs would be higher for asteroid mining but the marginal cost of expanding mining operations would be dramatically lower.

If you have already figured out how to mine 1 tons of asteroid ore, then mining 100 tons will require much smaller capital expenses than doing the same mining expansion on Mars.