How much does it cost to have valve stem seals replaced on my 420se on 1990 Mercedes-Benz 420SEL

My car is a 1986 420se and blows blue smoke from the exhaust after a down hill run

What You are talking about is an 8 Hour job plus parts.P.S. if the wrong person does the job and His "short cut" goes wrong and He "drops a valve" then it becomes a Major job.