I don't recall ever needing this much gold to simply progress in any of the Fable games, but were it ever necessary, it'd be doable.
I started a game of Fable 2 and progressed less than halfway through before starting this storyline, and you slowly accumulate gold from owned properties even when not playing.
I don't know when I last played, but prior to writing this, I checked to see how much I'd earned since whenever that was.
It was 949,300 gold.
Which brought my total gold up to 3,057,308.
I know all things are relative, but that's a lot of gold, I assure you.