cost for replacing main seals

Around how much am I looking to spend on replacing all the main seals at a dealer? Is this something I should be able to get done at any local machanic? The car has 150000 miles and I just purchased it and I noticed oil and crud under the intake manifold right on top of the motor and on the sides in the front of the motor. It looks like its coming out of the bolt holes. I'm sorry I cant be more descriptive and exact but this is my first 6 cylinder and I'm not use to the setup. I also think something is leaking on to the exhaust becase after the long trip home after I parked I had smoke coming from under the car in the front around the exhaust piping. What could that be?