Agreed that is probably the most interesting question. I've played the AH at a medium-high level (not anywhere near the top) with a 50k investment and made 500k in 10 weeks.
Even if somebody could have only managed that rate for all of Cataclysm only on one server, they'd still have made around 3 million gold. I know many AH players play on 3+ servers. Accounting for previous expansions, I'd say 20 million+ is very very possible if they've been doing it for 3+ years on multiple servers in multiple markets.
Yet I still see like "500K Milestone HOORAY" posts on TUJ and other blogs... so I don't really know.
Edit: Also gold cap is 1 copper short of 1 million.