There are at least 42 girls to get there. As with bosses you only see the girls you can now still get, so if you already won some then they won't show up there again.
And well that's only true for the "guaranteed girl"..you also have an additional chance of at least 1:118 at minimum if we base it on 1. all possible rewards from epic pachinko has an equal chance of being dropped and 2. there will be only the new legendary girl for event to get from epic pachinko. I personally doubt the "1." but well..we don't have numbers. That would make around 0,85% chance per draw and around 64.800 Kobans for a statistically(!!) guaranteed drop.
If we now take the best discount on buying Kobans we're at 420€ (4x100, 1x20) or 107,64€ with 36 months of waiting and gaining Daily Cards.
At least for me, that seems to suck. Given that there's a chance to get her without spending any money (you get Kobans from other sources then money or can defeat Ninja Spy) it's arguable but okay. On the other side I did around 500 fights for the last legendary girl and didn't get her. To statistically get her after 501 fights the chance would be around 0,2% per fight. In 72hours without spending Kobans you can get around 72*2=144+20=164 fights (30min per Combativity and +20 because you can already start with full combativity(given you can't increase that above 20 or reduce the recharge time)