I can't give any numbers or detailed info, but I can say that unlike western pop stars, J-idols are salary-based workers and already because of that they can't really be expected earning millions each. Also the longer you have worked with the same company, the bigger that base salary is, as Japan has that whole senpai, loyalty thing going on. And as far as I know, for extra work they get more (PBs, plays, CMs)...
I kinda like that a J-idol can't become a millionaire overnight because I think it keeps them humble, hardworking and grateful for what they have (lol does that sound horrible)... More down to earth because even if they are celebrities, they work for a salary like most other people... And don't go around buying diamond shoes or other absurd things.