I successfully purchased all of a ruling clan's (Caladog of Battania) settlements right at the start of a game (through the use of Developer Console) in order to test the feature.
I used the "autoresolve" feature (the shaking hands icon) in the barter screen. The first one cost me 5 million gold and the second, 7 million. The purchases were made through two different transactions. I have no idea if the amount asked was influenced by how much money I had.
Before the first transaction I had given myself 10 million. After seeing I'd need 7 million to purchase the second settlement, I exited the barter, gave myself an additional 10 million and came back to it. It still asked me for 7 million. I had a relation of 0 with Caladog since the game had just started.
