also, many will signup to attend and bail last minute
I've RSVPed myself with this intent plenty of times. I'd use a "maybe, please remind me" button if it existed.
Contrast Doodle.com, which offers a "yellow maybe" option intermediate between "green yes" and "red no."
There might be a sweet spot around $3-5 for that model.
Even better: charge $5 to book; attendants will be refunded the fee; any leftovers will be used for refreshments at future events.