The price required for an event is fixed per user, at the moment that user creates or enters the event.

So:

※ Once you have entered an event, no additional payment is required unless you cancel your entry.

※ Depending on the situation, the amount each user pays for the same event can differ.

Q. If an event was created during the free period and I enter it after the scenario became paid, do I need coins?

A. The price is fixed at the moment you enter, so in this case the scenario is already paid and coins are required. The event creator's price was fixed while it was still free, so the creator pays a different amount from you.

Q. I entered an event during the free period, and the scenario became paid the day before the event. Do I have to pay coins on top?

A. No. The price is fixed when you enter, so a later price change never asks you for more coins. However, if you leave the event and enter it again, the price is fixed again at that moment, and coins will then be required.