Actually they are very lightweight (about 12 lbs each).
If you compare them (wish I had a stock one right now with a scales),
you would find that a street hemi cast iron exhaust manifold (even
without the headpipe that goes back to where the header ends)
would probably outweigh the header by at least four to one.
The shipping weight for a complete set of Dougs or Hookers, even in
2-1/4" and including the box and hardware is only 26 lbs.