And there's some twenty million tons of gold dissolved in the Earth's oceans. Jules Verne made it the source of Captain Nemo's incredible wealth.
To put twenty million tons of gold in perspective, all the gold that has ever been mined by humans totals up to about 180 thousand tons. To put in another perspective: sure, it's gold, but at a concentration of thirteen billionths of a gram per liter of seawater it's worthless unless you have unlimited time and energy to extract it.
That's the problem with asteroid mining in general. Until the cost of changing an object's momentum goes down drastically it's not worth doing. If Pysche were a 1000 kg block of pure, refined platinum (market price: $34 million) you'd be hard-pressed to retrieve it and return it to Earth at a profit. Which is not to say asteroid mining is a bad idea; but first things first: you've got to reduce the price of interplanetary propulsion by a couple orders of magnitudes. One thing that never happens in a sci-fi asteroid mining scenario is the hero worrying about running out of gas. Propulsion in stories is always practically limitless and free of charge. Real propulsion will never be that good, but it could get good enough.