Well you're right and wrong. The costs of shipping would be great but the rewards are enormous. A single asteriod of 1 mile diameter has more iron ore than what was mined in the entirety of earth in 2005. Space mining will have to be developed eventually to sustain industrial civilization due to the coming shortage of rare earth metals. It won't be long before the planet runs out of it as population keeps growing and standards of living sharply spike across the globe.