In most cases, this is done a few months before the performance. They will have a lineup of bands who have agreed to play on site. At this point the promoter tells the band that they must pre-sell at least 60 tickets to get a time slot for the show. The better you do, the better your time slot will be, and in some cases bands that don't meet sales requirements are fired, without being reimbursed or compensated for their work. If a band sold 60 tickets at $10 each, the profit would be net