Using a mission designed to bring back a small amount as a baseline for bringing back a large amount is ridiculous, I agree. If I drive from Chicago to New York to pick up a piece of mail, you might say that one piece of mail is $3000/oz. But I don't have to spend much more to pick up a couple thousand piece of mail; I've already spent the money on the car. Adding maybe a trailer and the marginal increase in gas, you end up with... UPS. They don't deliver one package at a time.

Assuming the launch costs are the same or slightly more, how much extra will it cost to attach a rocket to an asteroid and slowly push it back to Earth orbit? I would wager that the engineering costs would be exceedingly more than the marginal cost of hardware/fuel. Couple that with keeping the resources in orbit and building new ships off-planet, economics of scale will easily take care of the cost issue.

One asteroid mentioned in Planetary's plan had more platinum than existed in the entirety of Earth. It's hard to imagine they wouldn't make money from that.