imho, i believe alot of the price has to do with how "correct" it is and how "complete" it is. i know it is like that for alot of cars, but even more so with a split.
there are some hard to come by split parts that can make or break the over-all look of the car. ribbed bumpers, correct semis, fenders, decklid, popesnose and doors are the bigguns.
i imagine sourcing parts would be tough to. possible, but time consuming and expensive.
the older the car, the more expensive it is to restore.
good luck & have fun.