My opinion is that a multiplayer-focused game should cost at the most 3/4s of what a single-player focused game does.
I mean, let's face it, multiplayer takes a LOT less effort to create than a well-made, or even decently made, single player campaign. Multipayer has only two aspects; gameplay and graphics, while single player has gameplay, graphics, story, soundtrack, and character design.
And a game like the new Call of Duty's, where everything is just recycled from the previous games with improved graphics, shouldn't cost more than 30 bucks brand new.
I mean, I like popping in CoD and shooting some noobs as much as the next guy, but realistically it doesn't have half the quality of Mass Effect, Far Cry 2, Freelancer, Aquanox, etc.