I saw that if you make a protectorate, you get part of their income. I made a protectorate and I didn't see any income, not even 2 denarii per turn. I remember from M2TW that vassals actually gave you extra cash of their treasury, if they got over 10000 denarii. Which means the AI will never ever give me money.
Except if I become really rich, gift 100000 denarii to the AI and then they will give them back to me. <=== don't underestimate that tactic, it made my vassals love me in M2TW for no expense from me, as I was getting back my gifts anyway.


Also: Do I get trade rights and military access?