How much does it cost to fix an air conditioner on a vehicle that only blows warm air?

It depends why it's blowing warm air. If it's low on freon you can get kits at auto stores or Walmart to fill the system. They're not expensive. Be careful though as you don't want to overfill the system. So best bet is to go to a reputable auto store such as AutoZone, Kragen, etc. If that's not the problem could cost you some big bucks to have condensor replaced. Cost will depend on the model car you have.