well, the overheating can be diagnosed.
are you getting compresion in the crank case? how bout the cooling system?
you should be able to locate the oil leak also.
that being said, say it is the head gasket blown and you wnated to up the $$ and rebuild the head. your still gonna have to take a close look at it.
the guides might be worn ect.
we charge 425 for a valve job but that includes the seat,valve work seals upgraded to a teflon pc type seal, tip height set ect.
then add another 100 or so for a surface of the head.
its goes up if the guides are worn and need replacement and or honing. same with the seats.
i would say that they are in line with what to expect.or real close.