A very wealthy friend of my dad explained classic car pricing to me one day. It is a combination of demographics and economics. At the time of my visit to his house he had more than $4 million in classics in his garage which included a true Shelby Cobra, two Baracudas, a Sunbird and 12 others.
Men (sorry ladies) tend to fall in love with certain models of cars when they are in their teens and early 20s. When they get into their 50's and 60's they have enough disposable income to buy their "dream" car. Prices climb exponentially based on current economy and availability of vehicle.
Then the owners get into their 70's and 80's and pass on. Their kids inherit the "dream" but most don't want it. Vehicle comes back on market for a quick sale. Prices drop.
Witness the price of 56 Chevys and 57 Thunderbirds. They are off their highs. Soon all those 60's Mopar vehicles will drop in price. Will they go to zero? No they aren't making Barracudas any more. But the days of 20% a year price appreciation are over for sure.
Broncos are one of the few collectible 70's vehicles (Trans Ams being another) and prices are climbing. Next on deck IMHO are the fox body Mustangs, Toyota MR2, or unbelievable the Fairmont (apparently good hot rodders). Not many want to collect the Plymouth Sundance, Chrysler Minivan, or Taurus. We should be good for a few more years. But as you see more obituary notices here in the forums the prices will stop climbing. Sorry to be a downer on that point.