The Normal world girls stealing from the event girls is good, not bad.

During events with high event girl droprates, the normal world girls leech off of these drop rates. You get these normal girls a LOT LOT faster by doing the events. Like 10 or a hundred times faster. And, they don't leech off of the event girls enough for you to not get the event girls either.

And the normal girls, who are cheap to upgrade and generate decent cash flow, are better than the mostly epic event girls who are very expensive to upgrade, may not generate good cash flow, and for the most part still don't have the stats to push free story girls out of the battle team.

As for events, after the first day you can basically calculate how much that it will take to buy the girl with Koban refills. The drop rate is fairly constant. In the case of Sara, its 600-700 kobans that you needed to spend. She is a 5 star legendary for hardcore who will be the alpha girl for any Hardcore player who doesn't have Golden Bunny.

I don't use kobans to do any kind of pachinko anymore. Getting girls by refilling stamina to free girls from bosses turns out to be cheaper and the Pachinko girls are 66% likely to not be in my class, 70% likely to not have stats good enough to push free girls out of the battle team, and too expensive to upgrade for the purposes of generating income.