There's literally 0 chance that mining anything on Mars and bringing it back to Earth would be profitable. Obviously mining is still going to be important to build things on Mars, but sending any metals over, no matter how rare, is just crazy.
Also, the price of platinum is currently driven by the knowledge that it's naturally scarce and the likelihood of a large amount of "new" platinum coming on the market is low. If this ever changes (we somehow figure out how to profitably bring back a tonne of platinum), then the price would likely crash, as the amount out there in space is essentially infinite.