98 Camaro

My daughters car, a 98 camaro, 6cylinder engine, was running poorly, missing and then it over heated. We stopped the car, waited for it to cool, then added coolant to the reservoir while the engine was running. Later that day, the car stalled on the way to our work, and wouldnt restart. We later got it started, (after about two or three hours) and drove it the half mile into town to a garage.
mechanic looked at it today, said the head gasket was blown, and that he didnt do that sort of repair. My question is: I know this repair is expensive and there could be underlaying problems, but how expensive normally? And what underlaying problems could there be?