Well, the question you need to ask is: how much players pay now enough to get the 3 girls for each event and how much players would be susceptible to pay for the event girl if the cost would be very low (for example 1event girl = 5$)

If the result is that putting 1 girl for 5$ give a bigger profit than now, so, just do it

People who paid 100$ doesn't have weight in the balance, they didn't get the money back with a lower price. They lost their money forever and prices change, that's all; it's just like people who pays an apple phone 1000$ and 6 month later, the same phone cost 300$. It's unfair, but it's life.
If you really want doing something for them, give them 20 chance to have event girls in the future.

And sorry about my bad english