Cheopis wrote:How big the belts are is pretty unimportant. The useful material content is what matters.
If an asteroid belt is formed by accretion of minerals that never formed a planet, it might have very few dense collections of heavier minerals.
If a planet formed, minerals settled, and than the planet was catastrophically torn apart somehow to form an asteroid belt, then there might be some very high concentrations of dense minerals present.
Earth's asteroid belt is something of a mystery. We know very little about it, how it was formed, and what the useful mineral content might be. We know enough to understand that there's potential for industry in the asteroid belt, with no doubt.
Not true. We know quite about about the overall composition of the asteroid belt--which tells us how much total iron we could find there, for instance. What we don't know is how it is distributed. We don't know where to find concentrations of specific elements, or how abundant such concentrations might be. But overall total amounts, we have a good handle on.
As Namelessfly says, the percentage of heavier elements (including most metals) in the asteroids is actually higher (on average) than in the Earth's crust. Much of the metal in the Earth sank to the center, leaving the crust metal-poor. So the crust is depleted in heavier elements relative to the average in the original planetary disc.