I don't think the mass is as important as accessible mass - let's say you can mine up to 4 km deep (the deepest mine on Earth at the moment) & you can't easily

I don't think the mass is as important as accessible mass - let's say you can mine up to 4 km deep (the deepest mine on Earth at the moment) & you can't easily do that any, there might be ocean, glacier, or even a city on top of the resource you want to mine. That limits you even further for terrestrial mining. Asteroid on the other hand are mostly just a few kilometers in size with a very few being bigger than tens of kilometers. That might quite possibly open much bigger mining volume than what's currently avalable on earth - rather than a big sphere with much stuff inaccessible due to being too deep, you now have millions of chunks floating around with a lot of stuff to dig through in the 4 km limit. No oceans, cities or glaciers to block you as well. And the arbitrary 4 km limit most likely does not really apply for asteroids - they are all long cooled down & the gravity is negligible so nothing should really limmmit you from digging hundreds of kilometers through Vesta looking for valuables. Also some findings indicate that processes that lead to dilution of minerals on earth might have taken place after asteroids formed, so it might be possible to find the stuff we look for in quite a pure and ready to use state.