Cost to replace a head gasket on a 250?

[Stock72c10]

I have been having a terrible time trying to track down a bad oil leak on the passenger side of my inline 250. I have replaced the side covers/side cover gasket, valve cover gasket and still it leaks. I now think the leak is coming from the cylinder head gasket. A local guy estimated it will cost $500-600 for him to replace it. Does that sound reasonable? How much more should I spend to have him just rebuild it while he has it torn down (bored out .030" over?). Your opinions are appreciated. I am planning on dropping it off after work tonight for him to look at it to be sure it is the head gasket.