I've noticed a good pattern to indicate car value. Junkyard availability. If you want junkyard parts for a car in my region, you're looking at good availability for 15 year old American cars, 10 year old Japanese and German cars, and 5 year old Korean cars. That obviously is skewed by demand and taste, but is a nice indication of when a car is too old to be worth fixing.

Also a good way to figure out what used shitbox you should buy next if you want cheap parts. :)