Demotion/promotion bonuses and game attendance are two different things, really. You'll have less season ticket holders and therefore less attendance if you demote (and vice versa if you promote). However, individual game attendance will also vary depending on the overall fan survey.
i had mostly 4-4,5 balls, as a champion but roughly 60k more visitor income next season compapared to a runner up season with 4 balls. Going back there to, or compare it to another teams of the league another small difference don't make nearly that much difference and behave nearly the same(maybe 5-10k each game) The same get reported from team who stay in the division, in the second season quite frequently that they get less money with quite comparable survey and don't get close to the value even with a good first season.
The same goes afaik for nearly relegated teams who make a turnaround, vs good team who are demoted.
You realize that the last game is pretty much the only thing that changes between each calculation of the fan survey, right?
sometimes those last game is also a tv and/or a rival game, the result of that don't have the huge impact on the fan support like the last game.(maybe when it is tv and rival togeether, but even then i would rate the effect lower)