A one-off 2013 Ford Mustang Shelby GT500 prototype used during the development of the upcoming 650-horsepower muscle car has sold at auction over the weekend for a staggering $300,000.As we first reported last week, this particular Shelby GT500 can’t be titled and licensed for street use.However, like many other one-offs, the car’s value comes from its rarity. The prototype vehicle was driven by…