IMHO they've now entered "Classic" status. Especially with more than 1 example changing hands for over 6 figures!
http://gmauthority.com/blog/2017/01/1987-buick-gnx-with-only-1500-miles-brings-in-117700-at-auction/
20+ years ago when I was in high school they were just old used cars. Now people are asking big bucks for some of the G-bodies while others are still dirt cheap even for clean ones.
My guess is nobody on this forum is sitting on a 6 figure GnX; but when a few examples of a car sell for a high price it brings the lower stuff up with it (most of the time). I can remember the 1972+ Dodge Chargers being practically worthless (A lot like how many people currently view G-bodies) but once the values of the 1968-71 cars started going crazy it brought the value of those coke bottle
B-body Mopars up. Chevelles were also a car that millions were produced and you can't touch a clean 2 door for under $20K these days (maybe a 307 Malibu; maybe...).
In another 10 years I wouldn't be surprised if you saw run of the mill clean G-bodies regularly changing hands for $20K+ (in today's money; not factoring in inflation). People will naturally gravitate towards these cars as the older 60s/70s era cars go higher and higher in value (if market trends continue).
Another factor is that guys that remember these cars as being cool are now starting to enter middle age. They've got savings accounts, 401Ks, Real Estate Equity and can afford a luxury item. I've been witnessing it happen with Fox Body Mustangs for the last few years. Their Dad's had/have the 72 Chevelle SS, but they want
Grand National, or a Malibu with a Blown LSx under the hood. Not only that; the Gbodies appeal to a very diverse crowd of guys! I don't know of any other car that is loved by so many different cultures. Low Rider, Dub life, Modern Muscle car, Drag Racers etc.