Some examples:
Canon FD 55mm F1.2 Ashperical. A year ago north of $4k, now less than half of that.
Olympus OM Zuiko 21mm F2.. same story. 2 years ago which is as far back ebays terrapeak completed listings allow me to browse shows one sold for $5200 while the latest sold for $2425
I think most that are into vintage glass for whatever reason has had their fill. I also think there is room to drop a lot more.
Personally I like it as I enjoy collecting old glass.