Not sure about cost, but replacing it is certainly a project. You'd have to separate it out from the floor pan, the area over the rear axle, and separate from the rear frame. You would want to secure everything and take careful measurements before attempting. It would be easier to assess if you have to replace the trunk floor - once that is out, you can see inside the frame and make a decision. On my 69, the frame rail had some holes, so I thought about replacing it. But without the trunk floor in the way, I was able to cut out the bad steel and weld back in some of the same gauge steel; a little welding and grinding, and you can't tell it was done. Plus, I didn't have to worry about getting everything to line up again.