Those resources already exist on earth, for which it'd be cheaper to just mine it out of the ground, than to spend the energy to bring ore back (then spend the same amount as earth mined ore to process!).
The one thing those asteroids have going for it is that it's already in space - so if you were planning on building a space station, space ship or something of that nature, you might be better off mining in space to produce the materials for it (and save the launch costs, for the one time cost of launching space mining and manufacturing plants).
The one thing those asteroids have going for it is that it's already in space - so if you were planning on building a space station, space ship or something of that nature, you might be better off mining in space to produce the materials for it (and save the launch costs, for the one time cost of launching space mining and manufacturing plants).