If your leather sofa cushions are sagging, there's no need to replace the entire couch. You can easily fix the problem with some new foam and a little bit of effort.
In the following YouTube video one user shows a very cheap and simple to way to try and repair your sagging leather sofa cushions.
Here's what you'll need:
1. Remove the old, sagging cushion from the couch. Separate some of the old foam covering so that the foam is accessible.
2. Cut the 1 inch foam to size, using the old cushion as a template.
3. Spray down the original foam with the adhesive spray
4. Place the cut one inch foam and add it to the original foam cushion.
5. Spray the other side of the foam and add the polyester cover
6. Place the new cushion back inside the leather covers and enjoy your sagging-free seating surface!
Note if this method doesn't work, I'd guess that maybe it's time for a new sofa. By the time you bring your cushions in for repair, the costs will likely exceed the value of the sofa.