They can give you an estimate of what needs to be replaced. It can be as simple as an O-ring that was pinched on installation, and gradually leaked the freon. If your car has lost all of it's freon, you'll have to replace the dryer (usually a silver cannister under the hood).

If there is a hole in the condenser, probably not too expensive. That's easy to access on most vehicles. The evaporator (in the dash) unit usually isn't that expensive, but there's a lot of labor to pull the dash to get to it. The compressor is usually the most expensive single item, but it can easily be accessed. Not a lot of labor to swap it.

The shop will either inject dye into the system, or put it on a vacuum to check for leaks. They can give you a cost estimate once they find the leak.