I know most hotels put these types of specials on for the guests staying there to try keep them in house and not spending elsewhere. I am sure you guys were unexpected and I don't think this event would have made any profit from your crowd, but overall a guest would not sit for 3 hours and be off after a few tapas plates and maybe 2 glasses of wine. Dumping is a problem world wide due to the costs of being charged if you have commercial waste. Rather make it free to dump otherwise the costs cleaning up don't justify the initial costs of charging to dump at a specific site.