Check carguru.com, cars.com or kellybluebook.com to get some estimates. The car looks goods, but you have to consider market conditions. Also, its 11 years old. $12,000 may be a good starting point. You may get lucky and a buyer falls in love with your car. They may be willing to pay more. Just my thoughts.
2006 GT, Strut Tower Brace, Gas Hood Struts, FR Intake Plenum cover, Metal Fuse Box cover, Moroso brake reservoir cover, Rear shock tower brace, Front Bumper Lip Spoiler, JVC DVD Head unit w/ back-up camera, satellite Radio and USB input, 2 - 12" sub-woofers, 4 - 6"x8" speakers, 2 - 6.5" speakers to replace the door subs, 3 amplifiers, LED lights under the hood and in the trunk, chrome e-handle on parking brake handle, chrome cup holder trim, and trunk lid cover with running pony emblem.