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.
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.
That might not be enough to break even.
The asymptotics work out much better with Asteroid mining.