I have 144 girl at the moment and still have no problems getting both drops in standard events without paying a single koban. The only times I had to pay was during two of the 4-day events (Legendary/Epic days), but even then that was way cheaper than spending 900 kobans for epic pachinko.

Could I have been that lucky for almost two years now? Maybe, but I think some people are just too impatient and start paying up after the event is halfway through. Keep at it, wait for automatic refills and don't pay until the very last hour.

And, yes, drop rates for standard boss battles are terribly low. I waited 3-4 months for the last Roko drop.