Why would it cost that much to transport it? You've got lots of gravitational potential energy to work with, in terms of shipping down to Earth, so it doesn't take the kinds of fuel that it takes to ship things out of the Earth gravity well. And you don't need to bring back the whole asteroid, just the products, so you're not shipping "massive amounts of rock through outer space".

Are you familiar with the companies that are being set up to do this, and that are backed by Eric Schmidt et al?