For most of comic book history, comic book values were dictated on the comic book itself. Demand, scarcity, artists/writers, 1st appearance, and/or death of a character were the traditional factors which a book would gain or hold value. An occasional TV show or Movie might affect the value as well. Then everything changed in 2008 with the launch of the Marvel Cinematic Universe (otherwise known as MCU).
The MCU’s official first movie was Iron Man and boy oh boy it was a home run. Not only was it a well produced and visually entertaining movie but it BROUGHT IRON MAN TO LIFE. This was the missing key for comics. Up until the movie, Iron Man was a sub-par comic character. Sure Iron Man had a long history dating back to 1963 (Tales of Suspense 39) but by the early 1990s it seemed that Iron Man would be a “B” star character, never fully reaching that “A” status of Spider-Man or Superman. With the hire of Robert Downey Jr. to play Iron Man, everything changed. Iron Man quickly shot up to that “A” status and so did the comics. Values have skyrocketed since. Silver age books that were moderately priced and available 10+ years ago were long bought up by collectors and flipped for profits year after year. For example, a mid-grade copy of Iron Man #1 from 1968 which might have cost $200 4-5 years ago, now will cost you $500-700 or more.
The boundaries between the screen and comics have become more blurred. 1st appearances of minor characters many times are sitting in dollar bins and then that character makes an appearance on a Disney+ TV show (eg. WandaVision). Not only do characters gain popularity from a show, but then Marvel brings the popularity back to the comics for even more profits. The universe that Marvel/Disney have created is really quite extraordinary and beyond rewarding for fans/collectors.
Independent publishers have seen great success in recent years as well. The Walking Dead lead the way, creating a decade long award winning TV show with multiple spin-offs. The success of TWD has also created opportunities for other small publishers/writers. Boom! Studios seems to have hit a home run with Something is Killing the Children and it appears geared to get its own TV show in the near future. Speculation that Something is Killing the Children could be the next The Walking Dead plus low print runs of initial issues have created a collectible gold-mine with issue #1 selling for $200-500+.
Since joining the Disney world, new Star Wars movies and The Mandalorian have pushed Star Wars popularity to new heights. Books published by Dark Horse comics over a decade ago have now come into the spotlight, since many of the popular characters were originally created for the comic books. The first comic appearance of Ahsoka Tano from The Mandalorian (previously from the animated TV show) was published by Dark Horse in 2008 and has consistently sold for $1000-2000 over the past year.
Bottomline is if you want to be a modern comic book collector, you need to look way past the comic books themselves and more to the future possibilities of TV shows, Movies, memorabilia and more. It truly is the NEW GOLDEN AGE of Comics!