According to me it will depend on the person buying the Mars soil. If, you are a scientist and would like perform some research on the Mars soil that you would love to shed those$20. But if you are a person who loves to collect unique/antique things that would be a good investment worth buying Mars soil. Whereas this Mars soil wont be much of a help to an ordinary man who is neither a scientist or a person who has a hobby in collecting unique things. Hence, overall buying Mars soil will drill down to the kind of a person you are and what would you do with the Mars soil - the cost wont matter if someone really needs it.