For $600 I would excpect new valves, guides and seals and a good clean up of the ports and shims done and new cam bearings. What is wrong with the head? craked?
If the head is in reasonable condition take the valves out and check for strightness.
Get the head crack tested and cleaned($50)
Deck the head to straighten(cost $30).
Get new valve guides ($100 tops)
Replace any valves that are bent cracked or burntout ($40 each but you shouldn't need to replace all eight).
Clean the ports and chambers up your self with a wire brush on a drill.
Re seat and install the valves and replace cam bearings ($40).
So as Jason has said do it your self and save your self up to $300. It isn't two hard but you will need to compress the valves. There are tools to do it for $30 or so but I just use a G clamp and some modified sockets
You will need a 18rg head gasket set with valve stem seals to suit the 18rg, 18rc ones do not fit but a 18rc.
But $600 seems a little excessive considering you could buy another 18rg with a good head.