Excuse me for being cynical, but if 105,000 gold farmers together have only 12 million gold, then each gold farmer has less than 120 gold inventory on average. So either these guys are selling the gold faster than they can make it; or more likely Blizzard only banned the bot accounts, and not the distributor account that holds all the gold.
I don't know what the current going rate for gold is, but it can't be far from $50 per 1,000 gold. So 12 million gold deleted sets the gold farmer back $600,000. And the 105,000 accounts at $20 the account cost them $2.1 million, money that goes directly to Blizzard. These bannings are quite profitable for Blizzard. The gold farmers will be back soon.
But through the law of unintended consequences, being banned is soon going to be a harsher penalty than before for a gold farmer. Because the maximum gold is earned at the maximum level. Raising the level cap to 70 means that each gold farming account needs to grind longer before reaching the cap and starting to earn good money. So every banned account is "out" for a longer time. Lets hope that this at least leads to gold farmers using less bots, as these are easier to detect than manual farming.