How mush will it cost me to add a bathroom in a old garage?

Is the garage attached to the home?

Is it insulated?

What is the floor made of in the garage?

How big a room do you want?

Is there access to the electrical and/or plumbing and where are they?

Are you in a cold climate during the winter months?

More info would be helpful.

Your biggest concern should probably be the plumbing. Is there already a rough in? If not you will probably have to get sections of the floor torn up which can get pricey.