Even gold, silver, and diamonds, etc., have no real value other than what someone placed on it. If everyone agrees something is of value, then it is. Like G-body parts. If I was smart, I'd have plucked up every 8.5" rear end I ever came across at what used to be giveaway prices. Smog dog 80s car rear ends didn't bring squat years ago. I can't tell you how many 8.5" rears I passed up a $150-200. Today- almost unobtanium and pricey AF if you do find one.
I've been into g-bodies since about 98' and 8.5"s were a hot commodity then, just more options available now but also more demand for things to support hp/tq than it was 20+ years ago. I can't imagine earlier in their lifespan they were easier to get or too cheap, a posi 8.5" was $750-1,000 back then from what I recall. When I found my first one, summer of 99' I had Turbo Buick guys and others interested(I posted about it on a Turbo Buick mailing list, if anyone remembers those things) and that was the price range but kept it for my 305 SS 😀. Some other parts have increased at a higher margin from our cars IMO. Not to split hairs, just never heard anyone indicate they were common or easy to get at any point. I did find a somewhat crusty open 8.5" for $250 about 20 years ago that I re-sold to a friend, that felt like a steal back then, why I grabbed it and just held onto it for awhile.