I've just used up quite a bit of kobans for a girl who hasn't dropped. so here is an idea how about if someone has 600 winning battles against an opponent, the girl auto drops. 600 combativity is 6450 kobans. this way it takes into account the time the girls are around which ranges from a week to 4 days, at most a person can probably use 40 combatvity per day so presuming a person wants a particular girl and spends all there combatvity on that one for a week. which is 280 energy subtract from the total 600, if the girl still hasn't dropped which happens on occasion, if they spend the amount of comvativity to meet the the max 600 combatabity it will cost 3440 kobans which is just a 2000 short of the epic panchinko which auto drops +10 other items. This is why I think its logical that 3440 kobans for an auto drop if they are focusing and 6450 if they are not is a good balance.
This is just a counter measure against the misfortune of spending that much kobans and still getting nothing, though its just an idea.