I need to check the math, maybe it's not balanced well, but basically it all depends on idol's fame level and salaries of other idols. For example, they are more accepting of low salaries if you're not playing favorites and everyone is underpaid equally. Or you can try lowering salaries of your top idols to make everyone else happier.
I'm starting to wonder how the salaries are determined... With my salary policy set to "Auto: Keep idols satisfied" I have one idol (with very high stats and 8 fame stars) pulling down over a million yen a week. A near identical idol with 6 fame stars is being paid (literally) 5% of that. Seems an awfully steep breakpoint...