Common to find cracked exhaust manifolds on diesels as well as pipes coming from the turbo outlet rotting away. Both of those need inspected carefully and addressed as needed since they are in the upper areas of the engine compartment and pose a risk of starting a fire in the enclosure under the bed in most cases. While you are inspecting under the hood, so to speak, open up your interior access and check from above also. You may find something you couldn't see from ground level.
I recently replaced the manifold on my ISC-350 Cummins at a flat rate shop and if I recall the price it was about $800 for that part of the job. I had some other things done at the same time and don't have the bill in front of me so I can't be positive, but it's pretty close.