The most I paid for a clan to join my kingdom was 100k gold, they have 3 vassals and 2 nobles. I remember having 50+ charm at that time and my kingdom was at war with them.
Looks like there are some other factors makes them join you for alot of money of sometimes for free. (Maby Hate/Love his King? How strong are your Kingdom? etc.)
I joined Vlandia in my current save and tried to convince "Ergeon" from Battania for my first time in the game just for testing, he agreed instantly after the convice attempt was successful, joined Vlandia with his whole Clan "fen Derngil" without a gift.
Some other Lords wants extremly high sums...
It looks more like a balance thing that they want so much money to change the side, otherwise you could just convince almost all lord to join you and you win. (Maby it should be possible with an extremly high charming skill, so that you can get on without fighting if you want)
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Making Money is too easy at all, it takes the fun out for me because it makes the game too easy, I don't know what to to with all this god damn money, I can't buy as much as I get from just one Workshop and my 3 Castles... It should be important to buy Workshops if you wanna make some real money just as it was in Warband... I get around 3700 Gold in a day for everything, my only Workshop gives me 655 Gold a denars the day.
I'm not even afrait anymore if I lose my whole Army because I just buy a new one, I could buy the best weapons and armors and almost everything else on day 240. Right now I have
320k Gold and I don't know where to put it.
The same goes for Influence, im getting 25 Influence a day and I own 2600 of it, I can put 300 Influence in the most Votes because there is nothing else to do with. It should be harder to get all this stuff early.