I agree with Patcher, but the model its what can be discussed, here is a way to do it...
Don't charge for basic online play and put a limit to either the time you can play for free or the level the character can reach, that way everybody can still play online multiplayer online for free, just not everything or forever.
Then you start charging 5 dollars a month for the possibility of using some special modes, play any amount of time or to reach any level, those users that pay every month get the new DLC maps for free.
There are other options to make more money out of the game...
Sell the single player game alone for less money (its usually a 6-8 hours long campaign) around 40 dollars seems fine, this should include single player and 2 player co-op. Then sell the multiplayer modes alone for another 40 dollars. The customer that only buy the game to play online will pay less, and the one that only uses the single player will pay less, only those that want both things will pay more.
Keep doing the same thing they had done till now, but tell Microsoft to rise the price the Live fee 5 dollars a year if the Live suscriber wants to play Call of Duty titles, and start charging the same 5 dollars a year to PS3 players. Its not a big raise so the players will be willing to pay it, and it could mean extra 50 - 100 millions a year, all for Activision.
