The shipping and handling fees are astronomical. Sending gold to Mars is very expensive. Sending gold from Mars is even more expensive. In particular there are no oil deposits in Mars, so kerosene for the return rocket is very expensive. (Hydrogen and Oxygen are very expensive too.)

Note that there is already a huge amount of gold in the oceans, but it's very diluted so it's too expensive to extract. You don't want to just know that there is gold. You want to know how much profit you would get after the gold bars arrive to Fort Knox.