A guesstimate to disassemble / remove the rear hatch, weld in a flat cover patch panel, prep, paint, clear and reinstall...$750-$1000, although I don't know the mount specifics of your model's hatch (dips, protrusions, bolting config). Let me know how close I got.
If cheaper...you'll proly be looking at a "Bondo" fill, conducted w/the hatch
in place, as opposed to a metal insert / hot work mod.
Where are you going to put your spare tire?
You can proly sell your old hard cover for a decent return too!