For a good body shop to fix that I can see $5k. Mine wasn't anywhere near as bad an I was quoted $2k-$3k. They would probably cut all of the old rusted metal out and make up and weld new metal in. Throw in some paint work and installing the rear window it is a lot of work.

Their is a seam back their that seems to be rusted out as well, cutting that stuff out is a big job. You could try and find a junked W126 with a clean track below the rear window and swap them out.

If you want a faster fix grind the heck out of it and get rid of as much of the rust as you can. Then POR it to stop the rust and use some copper flashing to build the lip back up. Use some good oil point as well.

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2006 CL500
1969 280SE
2023 Ram 1500
2026 Genesis GV70
2007 Tiara 3200