Here is the Neal Adams' cover for Batman #227...
It is, of course, an homage to Bob Kane's cover for Detective Comics #31...
Depending on your definition of what an "homage cover" is (basically, can you homage yourself?), then this Adams cover is the first comic book homage cover (I did a whole thing on it a while back here).
You see, when it comes to value, comic books are almost always based on two things, with the first one being the most important. That is "story importance" and the second is rarity.
The most valuable comic books are almost always the ones with characters' first appearances, major events, stuff like that. Then rarity comes into play, like a comic book error or whatever.
However, very rarely does "that's a cool cover" play into the value to any noticeable degree and yet in this case, non-graded copies of Batman #227 have sold for $201, $175 and $238 in just the last WEEK!
So this is really a case where the comic book just having a really cool cover by Neal Adams (he had been working on Batman covers for two years at that point, so it wasn't like it was his first cover) has turned the book into a very valuable collectible. As a sign of just how out of whack this, I did a search for the previous issue, Batman #226, and the recent sales on that were all between $4 and $20.
Thanks for the question, Duane, which took me on a whole other tangent!
If anyone else has a piece of comic book history that you'd like to see me write about, drop me a line at email@example.com!