MSP, you have a lot of mis-information in your post.
Ford determines the build mix between V6 and V8s, not the dealers. In fact dealers have to order around 2 V6s for every GT that they order.
There is nothing that prevents a dealer from adding things on to a GT, if you've read some of the threads here you would have see photos of body kits, stripes, hood scoops, wheels, etc. that dealers have added on to GTs and have sitting on their lots for sale, with HUGE markups.
Most V6s are selling below sticker, while most GTs are selling above sticker with NO dealer add ons. Presently, on the average sale, a dealer makes far more profit on a GT than a V6. All those dealer add ons to V6s are an attempt to make more profit $ on V6s since there are not enough GTs to meet demand.
The BMW 3 series starts at $28K and goes all the way up to almost $80K for a loaded M3 vert. That's a $52K differential that far exceeds your $13K example differential from the cheapest to most expensive V6 Mustang.