Gaskit issue can be diagnosed by seeing if engine oil is reducing, or if engine oil is milky or if engine oil cap is milky underneath, coolant reducing or there is oil in coolant or coolant in oil, is there white smoke , does the water fly out of the radiator cap when it is opened even when engine is cold and running,
if you have any of these symptoms then you have bad head gaskit, its typically caused if engine has ever overheated or has high milage, repair cost i don't have much idea, risk is that the you might have to get the head faced as well which is additional tourble/ cost plus the new gasket might not fit properly and may fail again in the future, and no it doesn't reduce car market value, if you keep driving with a bad head gaskit you will wreck your engine, but before replacing check for the above symptoms