The labor time could be about right but there are some unanswered questions.
How is the 5k dollars broken down? Fluids, spark plugs, filters, add-ons such as new belts, tensioners, etc. and sales tax can really add up quick. A high shop labor rate such as found on the east and west coasts can really add up also when multiplied X 16.
I would sincerely hope the definition of machined cylinder heads means that they were not only surfaced but a valve job and valve seal replacement was performed.
Details on the bill might help clarify things a bit.