It's worth whatever you can get for the engine and trans for the most part. Disc brakes swap onto the A-body cars (GTO etc.), all the suspension pieces will swap too.
I sold a pretty nice '69, knowing full well that the buyer was using the drivetrain and hacking it up to save a '68 GTO. Trunk floor, trans tunnel sheetmetal, gas tank and such all swapped to the Goat.
Nobody really wants to pay much for the '69 and up GPs unfortunately, most are used for parts rather than restored.
What's it worth? Not much.