Its all in the labor,and the markup on the parts (gaskets).Changing a head gasket isnt a 5 minute job,and then after the engine is put back together you need to purge the air out of it and check to make sure its not going to overheat/leak again.This all takes time,and garages get $60-80 per hour labor.You might work for free.....but remember,no matter whose time it is...time is still money.While you are doing the head gasket you cant be somewhere else doing something else...