Makes you wonder ... did DS9 have to pay Voyager to use the Emh and Dr. Zimmerman? Or did TNG have to pay to use Bashir? How does that work when its an in-house crossover? Did Joss get paid every time Willow or Buffy showed up on Angel?
It goes by creator, not by show, I think. Rick Berman & Michael Piller created Bashir, and they were producing TNG, so he was already their character to do with as they pleased. But the EMH and Zimmerman were created by Berman, Piller, and Jeri Taylor, and of those, only Berman was actively working on DS9 when those characters appeared, so Piller and Taylor would've gotten royalties, I believe. As for Whedon, again, the characters were already his, so I don't think anybody would've gotten an extra payment.
A better example is the
Stargate franchise. In
Stargate Atlantis, we've never seen Jack O'Neill and/or Daniel Jackson guest star in the same episode with Samantha Carter and/or Teal'c. That's because O'Neill and Jackson were created by Dean Devlin and Roland Emmerich for the original movie, while Carter and Teal'c were created by Jonathan Glassner and Brad Wright for
Stargate SG-1. So having them all (or one from column A and one from column B) would require paying royalties to three different people (I'm not counting Wright since he's a co-creator of
Atlantis). But keeping the two groups separate means they don't have to pay as many extra royalties for a single episode. At least, that's my understanding.
I guess there are some extra royalties being paid for having Munch as a regular on SVU. But CI is being produced by a cable network now and probably doesn't have as big a budget, so I doubt they'd pony up for having Goldblum play the Raines character. Especially since that show flopped and the character would be poorly remembered.