I know the tailgates that say "Super Duty" are worth around $400 on a bad day down here....they are frequently stolen for that reason since everyone with a 99-06 truck wants the new 'gate.
New from a store and used from you is two different things. Watch craigslist as this wll be your selling audence. Condition and what you are including with it (lights/gate) also adds or removes to the price. Assume that your location would also be part of it. Also depends if your tryng to get it out if your yard quickly or hope to make a few bucks and can wait for the buyer with the $$$. Don't know about your model but I have been holding out for a dark blue s10 bed and have found prices around 300 (just not blue so far)
Sorry i didnt see this but I'm way out in Oregon anyway. I had to settle for $750 from a local auto wrecker. It's an itemI didn't want to store or try to transport so I took the quick sale.
I build and sell custom truck bodies for a living. I can tell you 99% of the time it is cheaper to build a body like pictured above than it is to purchase a used factory body in good condition. They are certainly not giving them away.