I used 20% in my back-of-the-envelope calculations, but maybe...

I used 20% in my back-of-the-envelope calculations, but maybe I'm being conservative. In any case, I still found plenty of upside.

By the way, a small error in your calcs, there are 31.1 g in a TROY ounce, as is used for precious metals, rather than the garden variety ounce. I made the same mistake recently in value the silver content of some old pre-decimal coins.