The earth crust is estimated to be in the order of 10^19 kg. The 6x10^24 figure you mention is the mass of the whole earth. So your calculation if off by 5 orders of magnitude.
That's what Wolfram Alpha told me, sorry. But my $956 quintillion number is about as relevant as the article's $700 quintillion number. Even 5 orders of magnitude off, we're still not supply limited. The only number that matters is the $32/gram number.
