In regards to OA only, not variants, prints etc....Ross does great originals but the prices have gone too high for the average collector. Most covers start at 20k. Love his art but just can’t justify that much for a modern painting. And with the sheer quantity being nice produced, it’s an imbalance of supply vs demand. Often times you’ll see his pieces on dealer sites for below the initial retail ask.
For Campbell, his two biggest buyers have dropped out for a few reasons which has hurt his market. A Spider-Man cover sold for around $8500 at auction last month. A few years ago even if you were able to buy one, the asking prices were 15-20k.
Perhaps the sales of all other merchandise has made art sales a lower priority.