I think if this were my car, I would ask $26.5K and expect somewhere in the neighborhood of $24-$25K.
Its got a decent amount of miles on it and brand new track pack GTs are going for pretty damn cheap. Also this car's in service date will be important - if its an early 2012 in service, the bumper to bumper is getting ready to lapse within a year.
2011 Mustang GT M6 3.31s
Matt HONEYCUTT Tune, Lethal Off-Road X, GT500 axlebacks, J&M LCAs, UPR UCA, Whiteline LCA relocation brackets, Eibach Pro-Kit Springs, Koni STR.T shocks/struts, GT500 strut mounts, SR Strut Tower Brace
Best 1/4 E/T (05/18/14 MIR - no tune, X-pipe, or UCA): 12.920 @ 109.40 mph (2.059 60ft)