There were over 200,000 fastback Mustangs built from 1965-1968 and their prices have skyrocketed over the past 20 years. Supply and demand. When something's the hot vehicle of the day and everyone wants one, up go the prices. It is strange that 15 years ago you could get a decent running driving Bronco for under five or six thousand dollars. The price of fastbacks was driven by the movie Gone in 60 Seconds. I have no idea what's driven the prices of Broncos so high.
I agree with the statement about FB mustangs. Pretty interesting that a 20 year old movie could move the value on those, and then the values hold pretty stable for this long. There are only a fraction of those FB mustangs left also. That plays into it.
I think the Bronco popularity has a lot to do with the 12 year production run and virtually no changes to the basic look of the truck. A really wide portion of the population thinks of the Bronco as from "their generation". It spans my Dad's generation all the way to mine and beyond. Dad was wrenching on them in the 70's, and I was doing the same things in the 90's. Not the same as a Mustang. The 66 Mustang is a completely different animal from the 77 Mustang.
My generation finally has money (40-50 years old) and time, so back to the 80's we go with our tastes in automobiles. Apparently everyone wants to be 16 again and have the car we would have loved at that time, so a mere $78,000 takes us back to our youth. I don't think there are any better options from the 1970's or 80's (or 90's)