NAPA's CRC Brake Disc Quiet was roughly $3 a bottle, and it was more than enough for two cars.
It comes out as a thick rubbery goo, and dries in about 10 minutes. Once dry, simply reinstall the brake pad then wipe down the excess red goop that squeezes out. It's a little messy, but you're probably wearing rubber gloves when working with brake pads anyway.