1987 Pontiac 6000 How much to fix cat back exhaust?

A friend of mine has a 1987 Pontiac 6000 for sale, 142,000 miles, 4 door, 2.8L. The only work it needs is it needs the exhaust redone from the cat. Converter back. If I bought the part and just had the labor done, he said I'd need a "cat back kit, " or I could have an exhaust place use their own parts. How much am I looking at to have this done whether it's just the labor and I supply the part or if a place does it all? Thanks.