Well, Reproductions with correct appearance sell about 500$ now, Given a set of Factory installed 4 leafs are not dated and subject to use on 68/69 model year and folks seeking no compromises with the use of nice quality appearing Repops, I would suspect you are in the ball park at 500$ used. Finding past real world sales will be difficult in my opinion. I would imagine Condition would drive the pricing in a large way. They could require servicing and repairs, Hidden Broken Leafs, Sagging leaf's, Broken retention straps, Rust, Worn slider inserts, Mangled End eyes and so on. These would require serving & repairs. For a Car for sale depending on the "whole" of the Cars being sold the Remainer of the vehicle originally it could drive even the Choice to go pursuing Factory springs.
Does the Ends justify the Means.
My biggest concern would be the use of 200 dollar Generic 4 leaf's on any 68/69 Z.
Just thoughts as asked for.