Cost ideas are horrible, why have a limit on communication?
It's not like allowing a chat will diminish the Cow's income, who buys FSP for the purpose of sending a message on the FSBox?
If you are going to introduce a cost just to send an instant message with everyone else then who is going to actually send messages? The people who are wanting to advertise somethings. What is that thing people use to advertise things again? Oh yeah! The FSBox.
A chat should be a chat, no more. You shouldn't be allowed to advertise in the chat as again, that ruins the purpose of a "chat", therefore restricting advertising to the FSBox.