Of course its fake. It didn't come from Mars so it's not "Martian" soil.
Also since they cannot verify that their "formula" is correct and actually duplicates the soil on Mars, this is another reason it has be deemed fake.
They most likely received multiple, LARGE grants to undertake this project, now they are making MORE money off it? Well welcome to the US where everything has a price and can be sold.
Dirt... I mean its dirt folks. Scoop some out of your back yard say it "possibly" (to avoid those pesky legal proceedings by the offended Martians) Martian dirt and sell it for $15 / kg. Undercut them.
Personally I don't see the point. Its not a collectible, it didn't come from my Mars so you don't have anything special. How do we know its the final product? Did they mass produce this already? If so why? If not, then its probably one of the earlier "failures" and again is not real "possibly" Martian soil.
People will buy anything and this is proof of that
I don't know if this is real or not but I bet any person that enjoy delving into the sciences and space would love to have 1kg of Mars dirt in their house, including me lol!
Imagine all the conversations you could start with it, "Take a look at that recipient, it looks like dirt but its... well its dirt ok, but its martian dirt!"
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.