I've parsed this out several ways, and done the math different each time trying to figure out exactly how Event Pachinko will work, and I think all the scenarios I can come up with come to the same conclusion: I won't do even one single spin of Event Pachinko until I have every girl who is available in Epic Pachinko.
If they made it so I could have a reasonable chance of getting a girl for a few bucks, I would be POURING money into this game. But instead they make girls more and more expensive, and that stops me from spending another cent. Seriously, if I buy gold, convert it straight to kobans, and then spin 10x Epic Pachinko, we're talking, what? Over $30 US for a random girl? Eff. No. But $5 for a random girl? I'd be dropping $20-30 a month on this game without a second thought.
At this rate, the day my monthly card runs out is the last day I play.