It occurs to me I should share some of my personal perspective, beyond a process for how to determine one.
I wrote Mickey Mouse for Disney, as well as other characters. Comics and books.
I wrote them on staff; I think it's "fair" that Disney own them.
I also wrote them as a freelancer, for which I was decently compensated; I think it's "fair" Disney owns them, too.
I wrote a Gargoyles story for Disney Adventures, the concept for which was adapted into an animated episode, the ONLY time that occurred. I got credit, which they didn't have to give me. I thought that was nice AND "fair." I didn't get any extra money...because that wasn't part of my deal. Had I wanted it to be part of my deal, I wouldn't have been able to write that story and collect a pretty decent page rate. I thought that was "fair," too, if not nice, but it was their property. Could a case be made for me feeling they SHOULD have compensated me? You bet. DC Comics gets money for people who created DC character that are used on their shows. I think that's fair AND nice.
Now, there are always going to be examples of "fair" swinging two ways, and I'm not going to try to offer an answer that applies to every person.
But I will offer a method for every person to figure how much they think an offer is "fair" to them...
...and it will be significantly more complex than "I created it, so I should own it."
So, let's keep the conversation going.