The description of this achievement is slightly misleading. The achievement requires you to reach a cumulative total of 1,000,000 gold gathered; you do not have to have 1,000,000 gold at once. Progress is tracked by account, not by any individual character. All characters share the account gold purse, which caps out at 20,000. Make sure to empty your coin purse now and then so you can keep making achievement progress (I recommend spending it at the gambler to knock out the legendary garb achievement).