Keep in mind that the voice actors perform a lot of roles. Dan Castallaneta, for instance, has the following roles
Homer Simpson, Grampa Simpson, Krusty the Clown, Barney Gumble, Groundskeeper Willie, Mayor Quimby, Hans Moleman, Sideshow Mel and loads more
Or Hank Azaria, who has the following roles:
Apu, Moe, Clancy Wiggum, Comic Book Guy, Lou, Carl, Dr Nick, Duffman, Milhouse's dad, Disco Stu, Snake, and loads more
Virtually all the voices are done by three people, and then one person does Lisa, one does the male children and one does Marge and her sisters. I doubt the last 3 of those earn 8 million, and the first 3 do practically every single voice in The Simpsons.
Judging from that, The Simpsons has Voice Actor expenses of somewhere around 50 million dollars a year (8 + 8 +8 + 4 + 4 + 4 + 14) with 8 million to each of the major three, 4 million to the ones with few roles and 14 million to all the minor roles. That really isn't that bad for a show that repeatedly is in the top 10s or 20s for the most watched show of the year worldwide.
The problem is, most likely, that any of the 3 major voice actors are practically irreplacable. You'd have to revoice 5major characters, 10 minor characters and tons of occasionally appearing characters for each one of them. They have an absolutely amazing bargaining situation. The series practically cannot go on without them.
So the only trump-card Fox has is threatening to cancel it, but that would ultimately be something Fox really doesn't want, as it's still an incredibly large (and almost definitely incredibly proftiting) show.
