How much will it cost?

I have a 1990 3.0L stickshift Mitsubishi Montero with 147,000 mi on it. I have a leaking head gasket. How much will it cost to have the gasket replaced if i put it in a shop? I found this stuf that says if i drain the radiator and put this stuff in, the stuff i put in the radiator will fill in the leak in the head gasket. Will this help or should I have it fixed by a professional?