2002 Ford Taurus Brake fluid leaking

I was driving my daughters car and noticed the brake light was on. I checked the emergancy brake and that was not it. When I got home I checked the brake fluid and saw it was low, so I added more as I did so I pumped the brakes and saw that it was all leaking out at the rear driver side tire. I twas not coming from the brake lines running under the car but from the rotor itself. What do I need to do and how much will this cost.