In the late 1980s, after Ernie Ball took over Music Man I recall seeing used Rays all over the place for $3-400. Then in the 90s they were all the rage again and used prices went back up.
Popularity is always cyclical. 15-20 years is a whole new generation of players, right? Over that period of time I've been watching renewed interest in short scale basses, funky budget basses from the 60s being reissued, long dead amp lines resuscitated, etc.
Back then, a heck of a lot of people were dumping Pre-Ernie Ball Stingrays, and replacing them with Ernie Ball versions : the body contours were a huge improvement, and string-through was a real drawback, in terms of the availability of strings at the time, so the top loading bridge was also a big improvement (although that was already transitioning, at the end of the Pre-EB era).
Who knows, maybe there'll be a run on cheap regular Stingrays, as people replace them with the new improved version...