I have some more questions... recently, a lord in our realm took over a region. This region has 193 gold listed on its stats page. Well, on tax day, that lord made 356 gold in taxes, and the region isn't even at full production. How is that possible?
It must have been a rogue region. In a rogue region, there is no one to remove the gold from the tax coffers. It just builds up according tot he rules listed above, until a realm takes it, either through looting, or through TOing the region and then taxing it.
During the third invasion of BT, the city of Jidington was rogue for many, many months. When Kingdom of Alluran took it an appointed a duke, the duke collected over 7,000 gold on the next tax day. When Dwilight first opened, many realms made a LOT of money by looting the rogue regions around them. They were generating gold, but it was just collecting in the tax coffers. Several realms had official looting expeditions, and several people wandered the island looting cities and townslands.
That's one thing that made CTOs a bit easier back then. When you took the city, and the surrounding regions, too, you got a windfall in gold on your first tax day. I think most regions have pretty much been looted to death by now, and their extremely low population and production prevents the gold reserves from building back up.