Being a GT Turbo it's the last of it's breed (no more after 1984) and it was a rare edition to begin with. If it's in great shape and you're willing to sit on it while the right buyer comes along, you could probably put as much ask $6-7k on it and eventually get it. If it's just a "driver" no more than $2500-4000.
http://www.kbb.com and
http://www.nadaguides.com can give you the "book value" that a car dealer or insurance estimator would go by, but those numbers don't truly represent what the market bears for Mustangs at any given time. My 5.0 convertible "booked" at $1200 when I sold it, I got $3000 for it.
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1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4
tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".)