. So far as we know the bulk of that material is stuff that's easy to get here on Earth: silicates, sulfides, iron, nickel etc. Judging from meteors found here on Earth there are exotic materials like iridium, but in trace quantities.
Not at all. In a similar thread I linked to a USGS study on the prospects of space mining that showed that for an entire class of asteroids the average precious metals concentration is 28 ppm, with findings as high as 200ppm. In bulk, not concentrates, no overburden. I mean, that's insanely rich deposits. The richest gold mine on Earth is something like 40ppm - with lots of overburden. Most are 1-2 orders of magnitude less rich than that.
The problem with Earth is that most of the precious metals in the planet have sunk into the depths, with the crust mostly containing only that which has been deposited by later bombardments. But asteroids (with the possible exception of large ones like Ceres) are undifferentiated. Look at 16 Psyche, for example - it makes up 1% of the total mass of the asteroid belt and it's an estimated 90% metal. Ever seen anything like that occurring naturally on Earth? ;) Now Psyche itself wouldn't be an ideal target, it's a main belt asteroid, but still, it drives home how much these objects are not like Earth.
The platinum deposits in Canada's Sudbury Basin were delivered by a meteor
I think you're mixing things up. Sudbury is mainly mined for nickel - the platinum is recovered as a secondary product and is not the prime mining target (while not precious, nickel is a rather valuable mineral (nearly twice as valuable as copper), and Sudbury is one of the world's best deposits). And its minerals, while the result of a meteor strike, didn't come from the meteor itself. The meteor (now believed more likely to have been a comet than an asteroid) overwhelmingly converted to vapor and plasma and was blasted into the upper atmosphere and circulated around the Earth. The giant "wound" however, penetrated all the way down to the mantle, which bulged up and diffused with a giant pool of liquified rock and let to melt differentiation mineralization processes, creating areas of very rich deposits. The key issue is that overwhelmingly the minerals at Sudbury are believed to be terrestrial-sourced igneous deposit, even though the concentrations were caused by an impact.