browsing ebay and shop goodwill indicates to me that the overall trend is the NES (loose cart, loose console) is going down, and prices of DS are creeping upwards, and GBA continues to creep up while gameboy seems to be going down (except pokemon, naturally). this indicates to me personally that it's just the next wave of nostalgia, tho arguably we've been able to predict this with DS software for some time thanks to gamestop case gutting practises...
snes, n64 and gamecube especially continue to be very expensive with no end in sight, sometimes outrageously so (lol @ super mario world loose cart going for more than the cost of a quality paperweight).
i think most people who are nostalgic for NES and want to collect casually have already done so by now. CIB is an exception, but of course it is for ANY games packaged in cardboard boxes, they usually were tossed out or ended up ruined (bugs, moths, high humidity zones, unforeseen disasters, pets, etc...), but that wasn't necessarily what this thread was about so...
anyway i still find the price for NES consoles to be absolutely bonkers but it certainly seems to be plateaued as an overall trend.
edit: as a general disclaimer it's worth pointing out that i don't keep up with NES actively so this is very much watching from afar~