The economically fair way to do it would be to auction all seats at once, on a blind auction. Have the top 30 offers win, everyone paying the price of the 31st

The economically fair way to do it would be to auction all seats at once, on a blind auction. Have the top 30 offers win, everyone paying the price of the 31st offer (the first that doesn't win). You could probably do it in an open auction too. Count the people who want to participate at the minimum price, start incrementing the price at predefined steps and count the people who are still in. Once you have less than 30 people, you have a price. An open auction every month may be too much of a hassle, though. I live in Sofia too, drop me a mail if you like.