The transmission seal will run around $800.00. While you are at it, you might as well have the rear seals replaced too. The expence is mostly in the R&R of the tranny,usually around $500.00. Doing the rear seal will add about $100.00.
From exerience, I suggest you take any MB tranny to the dealer. Not only are they better equipped and understand what to do,they will warrant the parts for 1 year.
It's important to carefully consider if a re-seal is appropriate. If it doesn't shift perfectly, you might be wise to replace it with a rebuild. That will run you $2500.00,but will be cheaper in the long run if you plan on keeping the car.
84 300SD (sold)