Would you mind expanding upon this a little further? What sorts of cost's are associated with using Steam as a platform?
Steam doesn't discuss these details publicly. They work out pricing with each game's owner on a case by case basis.
That said, putting MWOnline through Steam could easily cost 20 percent of the game's revenue as overhead to Valve. That's a major consideration that would cut into the profit margins (and therefore the viability) of the game.