If you mean the rear main for the engine, the book calls for around 6 hours labor. The seal itself is maybe $20. I work at $40 an hour, so that would be around $250. I bet most places charge closer $80 an hour.
i paid just over 350 to have my tranny dropped and my have the transmission seals (front and rear) replaced. got the main seal redone to while the tranny was dropped. so anywhere between 200 (shade tree mechanic/friend) - 400 (rip off main stream shop)