Yeah, I doubt you get anything more than that offer OP. Mustangs are great cars but the market stays flooded with them because of their popularity. That all adds up to faster depreciation. Just for kicks, I did a search on just Cars.com and limited it to 11-14 GT and GT Premium. There are 1,468 listed for sale. Factor in the fact that not every person or dealer lists on cars.com and the number is even higher. Mustang is not alone though. As a rule, you have to keep a car a few years to break even. Trade fast and you will take a loss every time. I didn't get what I wanted for my 13 either.
1990 Mustang LX 5.0 Auto--Gone a long time ago. 1st Stang
1987 Mustang GT 5.0 Manual--Sold
2008 Mustang GT Manual--Sold
2013 Mustang GT 5.0 Auto--Sold
2015 Mustang GT 5.0 Manual--Sold