At the moment, Ford appears to be splitting the Mustang into five models – 3.7-liter V6, 2.3-liter EcoBoost, GT with the 5.0-liter V8, 50th Anniversary and SVT. There are 10 available paint colors, 2 of which require an extra outlay. Triple Yellow is $495, and Ruby Red Tint is $395. Regardless of model, navigation goes for $795, and reverse parking assist is $295.
For both the EcoBoost and GT, the six-speed automatic tacks an extra $1,195 to the sticker, and adaptive cruise control is the same price. The GT also has a limited-slip 3.55 rear end for $395 and a GT Performance package for $2,495. Recaro leather seats add an extra $1,595. For the EcoBoost, there's something called EcoBoost Performance package for $1,995. The system doesn't say what the packages include.
The info here also fits exactly with what we know about the Mustang 50th Anniversary Edition from the New York Auto Show. The package comes in Kona Blue or Wimbledon White with a special leather interior. According to this data, it adds $1,595 to the cost of a Mustang GT.
A user at the Mustang 6G forums also has images from the ordering system, and they include some info on the SVT model. While there are no prices, options include a track package, tech package and available Alcantara or leather seats.
Note, the pricing and options listed here could still change. That said, it's still fun to check them out. Information on colors and body styles can be downloaded as a PDF from The Mustang Source forum, and the rest can be viewed at Mustang 6G. Happy building.