I've had a lot of fun playing. I spent quite a while building up my finances, then pushed my own kingdom. I was enjoying slowly expanding, making a push and then buying my way to peace for a chance to restock and prepare for another push. Slowly I expanded, but it didn't feel easy.
Then I hit 225 Trading and was able to barter for towns and castles. Since by the time you hit 225 Trading you've probably already got absurd amounts of money, and buying towns dramatically increases your income further, within 30 minutes I went from my hard fought little corner of the world to owning half the map. I had to choose not to buy more cities because it was clearly making everything else irrelevant. I didn't need to build up armies for garrisons, because towns come fully stocked. I didn't need a siege army, because I was buying. I didn't need anything financial because my tax income skyrocketed. All other kingdoms were crippled without their towns. The only thing I had left to do was an incredibly tedious slog of conversation checks to recruit lords to my kingdom to fill it up.
Fiefs should require other fiefs in the barter. Coins can be used to offset negotiations, but a castle for a castle, town for a town, or maybe two castles for a town, etc. Raising the cost won't help because of the snowball effect of owning more towns, and the damage it does to the lords who sold their fief.