I'd say you'll be lucky to get $150 for it unsuspended. New was probably $5-600. I bought a 91 Avalanche with Rock Shox Mag 20 for $1000, list was $1090 if I remember correctly. I still have it, though I've since replaced fork,wheels, stem, saddle & front brake. There is no changing a u-brake, except with another one. It works though.

I doubt that XT der is original. Mine came with a DX rear; still works fine.