What does it cost and whats involved in replacing a gasket in the engine block?

I take it you mean the head gasket. Head gaskets are pretty cheap, but the labor is high. If it is a V 6 or V8 you may have to do only one side. A lot would depend on what engine you have. I don't know if it is a car, boat or what, so it is hard to say what all is involved. You do have to remove the head from the engine to install a head gasket.