Mass Effect uses SI units, so shouldn't it be 50,000 metric tonnes? That's 55,115.566 US short tons.
Also the price of platinum is in Troy Ounces (31.1034768g), not Avoirdupois US ounces (28.349523125g).
Which means 50,000 tonnes is 1.607537 billion troy ounces.
Well 1.6 billion is close enough then!