The bill in this case would be the taxpayers burden in Germany. That's what we pay taxes for (one reason), the police protecting our health and property. They could try to get some money back from the illegal party goers. At least those who don't follow police orders to leave the place. The police can give this kind of order. In case of public danger they can tell you to leave a certain area.
But the main point here is, that the call for a new law is just utter nonsense. We have enough laws to cope with such situations. There where illegal parties long before facebook and nobody asked for new laws. If such events didn't run out of control, nobody cared. If they did, most likely somebody called the police and they handled the situation depending on actual events and based on existing laws.