It's only worth what someone will pay for it - that being said, you might want to post it for sale in the classified section ( Mustang Classifieds - StangNet Mustang Classifieds ) and see if you get any bites.
Unless you target a Mystic owner specifically looking for that spoiler, I don't see it fetching an extraordinary "high" price, short of someone wanting it as a "collector's item" perhaps?
Ebay!! I sold a pair of used Mystic tail lights with the bezels for a 96 for 220+50 shipping. I had a lot of bidders on those. I usually only get around 130-150 for a pair of regular 96-98 tail lights. So...I guess the paint was worth an extra 100. Your spoiler in normal paint is worth around 50-75, so I say you will get around 150 for it on Ebay.