Re: \'X3\' Trailer
I doubt that's the problem. If some of the cast don't want to come back then there are still so many characters that they could make more movies.
Halle Berry, for example, probably commands a lot of money. So if she's too expensive, simply don't have Storm. No one would miss that character anyway (Storm and Rogue are, if I remember my childhood, the two characters translated least accurately from the comics. But at least this Rogue is interesting and charming, while Storm is not.)
The only three characters I can't see doing without are Wolverine (Hugh Jackman) and Prof X (Patrick Stewart). Even the villain magneto (Ian McKellan) doesn't have to be involved because our heroes can always find new bad guys to fight. Heck, even Prof X doesn't have to be in every film. Actually, being without his guidance for a flick or two would make an interesting story.