Roger,
Having moved, experimenting, through several 35mm SLR and RF systems and into MF, first with Mamiya 7 and now with Hasselblad C/M systems, I've been closely following used prices across a range of products for the last 6 years or so. What you say is generally true but specifically, often enough, not. Since the advent of adapters, first for Canon digital (especially the 5d I, II, and II) and later mirrorless systems (plus adapters now for digital M's, I believe) Leica R glass has doubled and tripled while the R cameras have tanked (I "invested" $200 in a non working R8 some years ago, sent it to Leica for repair, paid with tax and shipping like $575, all with a mind to sell, only to discover over the two years or so this half-assed project took me, R8s had gone from $900+ to five and change. Oops. You can now get R4's, of which I have one, for like $100. The only great lens in the system to hold its price and not skyrocket, for some reason, is the 60mm f/2.8 macro which is the only lens I have at the moment for my R.)
Similarly, M bodies have dropped in actual dollar numbers and in adjusted for inflation terms even more so. Not hugely, but notably. The glass, with the M9 and later, and with mirrorless systems, has climbed and climbed. Interestingly enough, the Hasselblad lenses are lower than they were but the C/M bodies have stayed the same and the more desirable wides and later F bodies are untouchable by wastrels and loiterers such as I.
Minolta prices -- the SLR system I started with when I returned to photography -- were rock bottom five years ago and are higher now for the stuff that gets written about -- the MD Rokkor 24mm f/2.8 for example, which I was lucky to get cheap back when, has at least doubled over the last five years, in terms of lowest available price. Same with the Rokkor 20mm and 21mm f/2.8's. The Rokkor 135mm f/2.0 now sells for $1000 -- triple the price for the same lens from Nikon and five times at least for the gorgeous equivalent Canon. It must be awfully good.
Nikon, the SLR system I've invested in most heavily, has its glamour babies (plain prism F, eg) and many specific lenses and bodies (the FM2n, the FM3A, where heart-swelling bargains could once be found, now are not to be had for less than premium prices) and certain lenses have gone up because of collective breathless reviews online. I just discovered happily that my Ai-S 300mm f/4.5 ED is selling for more than twice what I paid. Among the bodies, F's and F2's have gone up, while F3's, Nikkormats, FE's , F4's and F5's have come down, in some cases dramatically. For FM2's and FM3A's, see above.
Your category of "collectible" might cover all this -- but to me, what is or isn't collectible is a super-fluid notion heavily affected by two factors -- online chatter and adaptability to new digital technologies.