My advice: avoid protectorates like the plague. If you become one, you lose shedloads of money - even after the alliance is broken, which the AI frequently does. If you gain one, then at first you'll gain a ridiculous amount of money (thanks to the money bonuses) but then you'll encounter some annoying bugs on the finance scroll that you simply can't get rid of without starting a new game (AFAIK).

Protectorates are the devil, I tells ya.