PC games online are always free, except when you play some games like DoD or Runescape and be a member.
the only way it isnt free is to play against xbox360 owners, normally you can only play with PC users, but if you pay for a multi platform card or whatever, it will let you play against xbox users, though you get advantage, your using a mouse and have faster reflex and slightly more accurate aim, dont know, it depends if you're good in PC shooters, if not, your at the same level as everyone else then.
But you could always get a xbox360 controller for the PC, use that like your on a xbox, but you will need a GOOD pc to play this game. Not any old PC will do.