"A single platinum-rich space rock 1,650 feet (500 meters) wide contains the equivalent of all the platinum-group metals ever mined throughout human history, company officials said."
Quick Wikipedia/Google math:
1 cubic foot of Platinum = 1330 pounds [1]
Total platinum mined ever = ~25 cubic feet [1] * 1330 lbs = 33250 lbs = 532,000 oz
Current price of platinum = $1,560/oz [2]
Total value of platinum in an asteroid = $829M
And that's just the one metal, which is not the only thing they're mining. Throw the water in there for the space station(s) and it's not impossible to assume that one asteroid can gross well over $1bn -- we're not talking $trillions yet, but I bet they've modeled this out to make it a profitable venture...
[1] - http://www.gold-eagle.com/analysis/platinum.html [2] - http://www.kitco.com/market/
(of course this doesn't account for the fact that if you suddenly double the worldwide supply of platinum overnight, the price likely won't stay at $1560/oz....but still...)