It all seems like a huge donations scam to me. Sure, they have some costs for the utilities, toilet paper, decorations, etc, but it's definitely not 15,000 Euro per day.
You understand just fine. They choose a number for what they hope to get from that assembly and always always always announce a deficit at a point partway through the event, so that people will dig deeper in their pockets and give more.
Even if they get twice the chosen target amount, it is sent away to Watchtower and at the next assembly, they only had a tiny bit of operating costs on hand, so they start at a deficit again.
Apparently, the deficit scam has been going on for decades, all around the world.