My understanding is that most of the costs were already committed in Feb/March, before the original summit date, which has since been moved. If that’s the case, cancelling now simply means wasting money that has already been spent, so you may as well give the opportunity to leaders who are doing the work and can benefit from this development opportunity.
It looks like around 60 people attending - so at least get your facts right. 100+ and 500k seems so random and guessed.
Also 200+ - where is this from? We are fat around the middle so you should cut more. Be realistic people.