Assuming you're already the king/queen, which you will have to be, the Sunset House Demon Door will provide with a free 1 million gold. If you haven't been there, it is locked from fast travel and you must go on foot. Quick travel to the MourningWood Fort and exit through the back door (the one opposite of the town) and stick to your right-hand side. After walking for a bit and fighting off waves of hollowmen, you will reach the Sunset House. Upon entering the area you must travel to the left and there will be the Demon Door, whom will accept you into his portal, allowing you to open a chest with the 1 million gold. Do not yet deposit the gold to the treasury.
After this, you may need more money (you're doing the good options, correct? You'll need about 7-8 million or so gold to save everyone, I believe). With the 1 million gold you have gotten from the Demon Door, purchase all of the businesses from each town and (if you want) you may rent out a few houses. Every 5 minutes you will earn the gold from each business you own, and each business gives you a few hundred gold, so you will be making thousands of gold each five minutes. Play for a while and do what you'd normally do for gold (possibly a weapon that gives you gold when you kill enemies or a job?) and after a few hours of game time you'll have a lot of gold.
Also, might I mention that you shouldn't make 100% good decisions during your rule. I made about 3 or 4 bad rulings to earn a lot of treasury money and I still earned 100% good and the people still love me. I would suggest breaking your promise with Aurora and denying everything positive about it, I mean, you can just go back there and do one or two quests for them and they'll like you again. I also turned Bowerstone Lake into a mining facility area, but you don't have to because I believe that if you keep Bower Lake full of water you won't lose any gold, you'll only earn some if you take the bad option.