With the desire to offer an upgraded server and enhanced security, we are passing on the cost of this server as a small rental fee to users of EventCORE. The rental fee is based on the total number of attendees associated with your event.
The actual costs are listed in the EventCORE Knowledge Center in the article on "Hosting Fees".
You can always check your current service fee by going to Admin: Setup -> Start. Go to the section titled, "Support and Billing Options" and click on the Current Bill link.