I recently had this discussion with my cousin who has a bunch of older bikes he was thinking of selling. About 25 years ago I started collecting a few C&Vs now down to only about a half dozen older bikes. Everything has tanked, especially mountain bikes. 20 years ago you could sell a rough cromoly frame easily for 200-300 with many bidders. Nowadays you're lucky to get 1-2 bidders and $50...

Think you have a nice titanium moots 26r bringing in big bucks, think again....
Think your carbon bike with USPS logos is worth a bundle, think again.....
Think your hip handmade NJS kieren bike is real expensive, think again....

Luckly I've sold the first two types but still have two NJS bikes most cause they are still pretty cool to look at, fun to ride in the city and locally and not worth much

Prices spiked up with the Lance era for obviously reasons, everyone loved bikes and couldn't get enough of it. GenX got jobs, bought the holy grail bikes they always wanted as a kid, like David's Masi or a Klein Top Gun. But with lots of collectibles its the general masses who moves the needle of prices in the short term. Should a common $1000 bike from the 80s be double the price in 25 years (supply and demand pricing)? IMO the prices have dropped and leveled off to where it was before the boom. Just like you should not be buying an air cooled Porsche at the moment or a convertible 70s muscle car at the current price peaks. Sorry if you've been buying and holding bikes for the last 30 years and need to sell now. Expect to get fleeced.

Good thing if your looking, the price is right now
Want a 3rensho for under a grand?
Want a Merlin for under a grand? These are 2019 prices and probably won't change much for some some time. The casual collectors have moved on to something else...