Edmunds, KBB, & NADA Guides are reasonable estimates. But shop around with dealers for the best deal or try selling it yourself.
When I traded my 2005 GT convertible for a 2013 GT Premium Convertible, the difference between (2) dealers only 25 miles apart was almost $6,000. And it was all trade in value as the net purchase price of the new car was within $200 without the trade.
Last year my son traded in his Mustang for an Escape because they needed more room for their first baby (stroller, car seat, etc). Obviously, he misses the Mustang, but is very happy with the Escape.
2013 GT Premium Convertible
Sterling Grey/Saddle Interior
Automatic/Brembos/Shaker Pro Audio