This doesn't pass a basic sanity test.
Check your units.
Er sorry, 21M and $350,000 respectively.
What about my units specifically?
You are saying that all the gold that has been mined would fit into a ball the size of a melon. There is much more gold than that.
Start with your number for the total tonnes of gold from your reference. Then construct the conversion factor. The way I teach this is to write down an equation of two things that are equal, such as
x tonnes = y kg
Then, for this problem, you want to cancel tonnes. So divide both sides of the equation by "x tonnes" and there you have
1 = (y kg / x tonnes)
Multiply your number of tonnes by this conversion factor. I think you will end up with a cube about 0.1 or 0.2 Km on a side. I can believe that number.