This game need money sink. But not the price on 5*, because this unleash suffering for the big harems. And we already have pachinko problem for the old players.
Long time ago I said about better way - "golden" girls which you can bribe in your harem for the very high amount of money (10 mln for the first, 100 for the second, and so on).
Or send cobans for this.

PS: Actually "special" girls like this is a good way to spend unused quest energy.