Well, he can offer 50K and HOPE that no one around your area with deeper pockets offers more. I say offer 45K and tell the dealership to stay in touch if someone offers more. If your dad is REALLY lucky, he might be the only one who'll bother putting in an offer......and then it's his. Would probably be the cheapest GT500 bought anywhere if he can pull off 50K or less for it. Good luck
P.S. If he does get it, and has any intentions of selling it in the near future.......tell your dad to keep the mileage low and the paint as perfect as possible. I don't know what someone will be willing to buy it from him for.....but don't count on getting more than 5-10K over what you pay for it. And that's assuming your dad nails the Shelby down for 50K or less and it stays in prestine condition.