A well equipped 1991 LX 5.0 Mustang MSRP was $14,055. In 2016 dollars that's roughly $24,764. Still 10-15K less than a V8 car today.
they are very far from equal in the equipment that comes with the car.
Near double the power, bigger brakes, wheels & tires, airbags all around, better fuel economy, etc, etc