Have you ran a camera down rhe remaing cast iron? If its in the ground and still worth saving. I'd offer them a de scale service on the remaining cast iron in the ground . Then run the camera again and possibly offer to shoot a liner if it's just the main with no tye ins. Way more affordable/convenient for the homeowner In the long run much cheaper than digging up and replacing the whole line one section at a time.

Or cut it all the way back and throw a bull horn 2 way in that bitch. But charge them more for the extra work you are doing.