Being king and maintaining a kingdom can be quite expensive if done correctly. Vassals are expected to pay their king tribute in return for his protection and favor.

A tribute is paid automatically by all kingdom provinces in gold. The king receives tribute every turn from every kingdom member. The size of the tribute is proportional to the province's networth, so larger provinces offer more tribute. If you are king, you can see the tribute in your end of turn screen near the military section.

Vassals do not have to pay tribute out of their own pocket. The tribute is generated automatically and is independent of that province's finance. Basically, the king will receive money for each province that is in his kingdom and this money will not be deducted from the vassal's account.