There is a perk from the trade skill that allows you to buy towns & castles for money. A town can cost several millions, so it's useful for easy conquest
You have to hit 300 skill in trade and that take quite a bit of doing.
As for the money you make in game, I finally figured out why you make so much of it and that is how costly it is to actually run a kingdom. Generally speaking I always play as a Vassal, never a King so never understood how much money you actually spend as a King but finally took the plunge a few days ago and let me tell you, 2 million gold went away in a heartbeat.
First you have to "Bribe" lords to join your faction. If you have really good relations, the cost isn't that bad but unless relations are really high, it costs an arm and a leg. I recruited all the lords that had made my friends list because I needed lords as quickly as possible. I think an 80 relation cost me only like 25k but to recruit a 15 relation, Tier 5 lord cost me like 500k, a Tier 6 15 relation cost me over a million and as your kingdom grows your going to need more and more of these lords.
Then you have the cost of ending wars to consider. If you barter to end a war, and you will probably be forced to to it at some point just to bypass the council, it can cost huge amounts.
Then you need a pretty big treasury saved up because you will lose workshops, caravans and fiefs and find yourself operating in the negative while you try to get everything back under control.
I think I am finally starting to see that income is and has always been balanced against the logical end game, owning your own kingdom.