I have the same car--81 300D non-turbo. I replaced the valve seals myself several months ago. Cost of the seals from a non-dealer shop was about $12. The job took me--working slowly--about five hours. Having the offset wrenches for adjusting the valves helped doing that last stage of the job. I don't know what a shop would charge. After the seal job, I still am using a qt of oil every 800-1,000 miles.