Do you have to pay when joining the playstaion network on the PS3?
No, PlayStation Network is free to join. Some of the content of the network, like games and movies do cost money.
You obviously do not own a PS3. The Playstaion Network (PSN) comes free with your purchase of your PS3 console. All you need to play online is an internet connection. It could be DSL, Cable, etc. Black Ops is no different. No game on PS3 cost anything extra to play online, only the game itself. I hope this clarified your question, and if you have anymore, please don't hesitate to ask.