Lenses seem only on the up lately, but depends what you go for, the best are rising.
Takumars and FD lenses were getting junked for almost nothing 5-10 years ago. Now good luck getting anything (except maybe a 50-55/1.8) under $100. I've seen FD 85mm f/1.2L lenses go for $800, and version 1 EF 85/1.2L for the same.
Leica M seem to follow what some of Canon's white L lenses do. The new models come out for double the price of the old models, then the second-hand old models go up in price to halfway between new-old price.
Mirrorless/MILC/EVIL/deck-of-cards-on-a-beercan cameras are certainly not helping, especially with the digital video crowd. Even absolute-junk C-mount lenses that nobody wanted anymore a few years ago have found homes on Pentax Q and Nikon 1, for those going for that 'retro' look (whatever that is).
Medium and Large format are hell cheap comparatively. Like brand-new Mamiya 645AF lenses are $2k+, the same lens you can get on fleabay for $4-500. cf Canon L lenses, I've seen them second-hand for more than B+H prices. And large formats might still be worth a bit (up to a grand or more for the newest apo/xl versions, but still fractions of what they would have cost new.