Jim Starlin is the creator of Thanos and the Infinity Gauntlet comics, which Kevin Feige took a lot of inspiration from for his MCU. A while back saw Starlin mention he had come to a mutual deal with Marvel over the rights to Thanos, which also saw Starlin write new Thanos comics.
Starlin also created the DC Comics character Anatoli Knyyazev, aka KGBeast, played by Callan Mulvey in Batman vs. Superman.
Starlin posted the following, via FB:
Just received a very big check from D.C. Entertainment for my participation in Batman V Superman, Dawn of Justice (Anatoli Knyyazev), much bigger than anything I've gotten for Thanos, Gamora and Drax showing up in any of the various Marvel movies they appeared in, combined. Guess I'll finally have to sit down and watch the movie.
You will note that Jim Starlin mentioned he hasn't seen Batman vs. Superman yet, with the reason being because WB stiffed him at the Batman vs. Superman premiere. Starlin posted on Facebook a while back (since deleted) that when he showed up at the Batman vs. Superman premiere, his name wasn't on the guest list and he was turned away, even though he received an official invite. Starlin wasn't too happy about it, and he even recently blasted Suicide Squad, calling it "garbage."
Regarding why Jim Starlin received so little for Marvel Studios using three of his characters (now thought to be four), if I had to guess, I would chalk it up to Marvel CEO Ike Perlmutter who is known for being notoriously stingy. It's a shame Marvel didn't pay Starlin what he's worth for using Thanos, Drax and Gamora, which are three huge characters in their movies.
Maybe the next time Marvel Studios wants to use one of a Starlin's characters, Starlin will hold out for a bit more? I also hope this means the "wounds" between Starlin and DC are being healed as getting Starlin involved with DC Comics' Rebirth would be damn cool.