I probably paid about $30 for my Ibanez CS9 stereo chorus new in 1981- now they go for over $100 if you can find one. There are several variables: price goes up if a famous player uses it, when paradigms shift (e.g., from tube to solid state back to tube), a pedal gets good world-of-mouth, and if a company quits making them. All these things happened to the CS9. Prices drop if a pedal is unreliable, if the market is flooded with them (e.g., Boss SD-1- great pedal, but cheap), and when a company comes out with a truly improved version.
I bought a '66 Fender Princeton Reverb from a girlfriend for $25 in the early '80s. I've carried it around for many years, not giving it much thought, until I found out that that they have become very popular among blues and studio guys. They go for $1600 these days. Bought a late '60s Ampeg B-15 as my first amp in 1977 for what I could afford- around $150. Then gave it away 15 years later. Before everyone figured out who James Jamerson was and what he was playing ('66 precision for $350 too).
There's really no way to know how these things will turn out, though. I have an old EH Dr. Q that I'm still waiting for it to go up
