might i also add to this (i know i am 4 month late but whatever), it also depends on the area you live. For example in California, specifically Southern California, prices for cars are ridiculous, and yes prices for mustangs are going up. When i was 10 years old, my dad bought my '72 coupe for a whopping $800. Fast forward to 2012 when i started modifying it,prices for the 71-73 were really cheap. You can get a running coupe for about $1000. 64-69 mustangs were less than 15k at that time in very decent condition. Fox bodies were cheap and so were early sn95's. Fast forward literally 3 years later, you'll be lucky to get a 64-69 for anything less than 25k. Even the 71-73 are getting expensive, a CL search will show a shitty, but running 1972 mustang for 2600 at the cheapest, and a fastback going for 20k. Fox bodies, 86+ are going up, now you'll be lucky to find a fox for less than 5k. I bought my 94 last year for 2100, at that time you can get one for 1200 at the cheapest prices are now above 2k for a v6. one thing i always said to people, as previous generations get pricey, people will look to get the oldest cheapest generations. and as more people are buying the older cars, the prices go up. And i specially want to blame the people who flip cars, they are the main reason why prices go up, and the stupid people who will pay that outrageous sum. For us who have ours, we can careless honestly because we already have it, but our cars become targets for thieves. Although, it is becoming harder to sell stolen vehicles, it doesn't stop the thief.
Also want to add that my girlfriend bought a 1975 280z last year for 2300 not running, prices are now upwards of 5k.
Theres always going to be a fan base for these cars, and while websites which are stance nation, canibeat, and cars meets and car shows become more and more popular, people will see how good these "undesirable" cars can look and they go out and buy one (like some people mentioned). Hell, going to the beach in my 1972 i got stopped by some older dude and told me "i never liked coupes, especially this years, but your car is making me rethink that." It also brings more attention when i roll in with my lumpy cam, and they see a 21 year old come out. Makes an imprint on them and they remember it. THey probably wont go out and buy some, but theres always that one person who gets inspired, or that 15yo kid that tells his dad thats the first car he wants.