did the head gasket blow out from overheating, or is it just leaking? if it was badly overheated, you have to consider an additional cost you might not have included - having the head machined down to be flat and true again, as they tend to warp and will never seal properly from just doing the head gasket job himself.

often when people quote side work, they don't always get into details of what other costs may be involved. regardless, $600 labor on that big of a job is worth paying if you don't have the time, or proper tools to do it yourself.

just factor in more cost for machine shop work on the head if needed, as well as the wiring harness like someone else suggested. nothing worse than getting into a project like that without having enough spare coin set aside to finish the job in a timely manner