Although the prices on ninjas differ for each event, it seems like they have a range in terms of the pricing. Like what Tobei said, I recorded prices for certain ninjas and some events is cheaper compared to others. For example: DPS hidan was cheaper in time-limited ninja recruitment compared to other events like lucky board and missions. It makes sense since you only spend 10 coupons for a key and you get only points and not items. On the otherhand, LB and missions costs 20 but it gives out rewards while you rewards while getting points.