Curiosity cost $2.5 billion. Round it up to 3 since it's a round trip. You'll have to bring back 200,000 pounds of platinum to break even. That's like 3 or 4 Sherman tanks.
It will get cheaper as we get more reusable launch vehicles!
Resource prices will also drop in response to a larger supply. That's what I was taught once upon a time. Now of course you have the likes of Goldman Sachs manipulating prices by stockpiling resources to lower supply.