Just to make things clear
When Mayumi said about she's not even make ¥50,000/month she's not talking about her DVD payout
For 2-3 days shooting for a single IV the studio paid her a lot (compared to what you'll earn with 2-3 days regular office work)
How much she’ll earn for 1 shooting for 1 IV are depend on her popularity, sales record of her previous releases etc, but I’m guessing she’s at least got around ¥3-400,000 one time payout per DVD shooting
The studio have to spend a lot of money to pay the idol, various staffs, accommodation, production cost etc, they have to pay all that months before they start to make profit
Because of all that, the studio usually is the one who have all the right for the material, music and movie industry also have the same system
For example if a singer change record company, new record company can’t just reprint the singer old song without some agreement with old record company
Why is Mayumi didn’t get any commission from her sold IV?
Even Mai Shiraishi need a full year to sold 228,783 copies of her PB, Mayumi is not Mai Shiraishi and apart from her one time payout she’s unlikely to get reprint bonus from her DVD
If Mayumi’s new IV sold out and need some reprint, not only she’ll get some bonus money, she’ll also will have better initial payout next time
This is how things work in the industry