Depends on what you want...
When you are a slave you actually give 15% of your income/turn to the slave master
In return when you are attacked 30% of the time the slave master`s 5-6 stack will come to your rescue. Thats if he or she has so many stacks yup.
If you dont like giving your money away then You got to be 10 times he`s Pr/Power and then click "Slave Empires" -Click "Declare Independence"