And what's the minimum that you need? Not that I'm in a position to visit for the event, but do you have any way of making the event happen if you don't get to your magic number? I mean something like "We'll see if those that have signed up are interested in sending in a few bucks extra (each) to make sure the event comes off as planned" or "Well, we could go without spending the money for the surveyors and just use a yardstick to measure off the distances" or "Instead of having an ambulance standing by we'll buy some new band aids and a fresh bottle of mecurochrome".
I'll hope those last ones aren't your choices, but - is there some way to get the event to go even if the required 50 don't sign up?