The gold that they make the masks out of come from the coins meant to pay Athreos for passage to the underworld. That sort of wealth adds up over the years until it's kind of worthless, but the clay funeral masks aren't as resistant as... y'know, metal, so they end up being used as money in the underworld while the gold continues to just stack up.
Ohhhh. Is there a source for that? Either way, I believe it. Then I still wonder what's up with King Macar.
I also wouldn't mind some insight into upon what planes Nekusar and Oloro dwell on.
More to current events, wonder how Jeleva is holding up against Emrakul and her'akul influence on Innistrad.