If donations via an XBL friend aren't possible, you can still reach the 6.5 million mark without too much hassle. Upgrade your working skills to level 5 (you get insane amounts of money that way) and invest in property, jacking the prices up for a bit. You'll get insane amounts of money in no time at all and you can keep donating it to the treasury. When it's time to make decisions, don't bother with the Auroran's promises or Paige's - it'll save you a packet. Also donate the proceeds from two items you have to collect. I think I only agreed to the Dweller's requests and the Auroran outpost but otherwise, everyone else got the evil side of my choosing (not just the promises, but all other requests/decisions) and because I had filled the treasury enough, I was considered an awesome wonderful monarch at the end. Use all the money in the treasury for investments until you have enough to re-fill it up to the required amount.
Of course, if you've already ruled on everything except the final decision before the last battle then you're a bit screwed unless you have enough to invest in property and can hang around waiting for the money to pile in. But I did it without Gold and without someone else giving me money so it's not hard, just takes a bit longer.