6,500,000 (6.5 million) needs to be in the castle treasury. 8.5 million is the total that you need should you chose all of the "good" choices which deduct from your kindom's savings but keep your promises and make the kingdom love you. This is also assuming that you gave the valuable quest rewards to the treasury, and not yourself.
Should you choose the smarter choice of keeping the drinking limit as it is, the people will not randomply puke in the street from being drunk after all is said and done, and your total required drops to 8.4 million without causing either side to dislike you.
This amount needs to be saved in the treasury by the end of the day marked 121 when talking to Hobson and turning in that quest. The easiest way to do this would be to buy every bit of property you can from the beginning and make all prices set to "Highest". Doing this will earn you money pretty quickly (around 150k each payoff at the end when you own all of Albion and Aurora.) This will cause the villagers to hold you in a negative opinion, and if you leave it for long enough some will come to hate, and eventually fear you despite an otherwise pure record. Their opinion is easily fixed by setting all prices to low when you have enough, then consequently saving the entire kingdom WITH your gold. Saving up 5 million in your personal vault before donating while doing this is also the easiest way to get the key for the treasure at the bottom of your gold pile provided you donate EVERYTHING to empty it again.
For more tips, see Weight of the World