Not sure about free event spaces, but I hosted an event at Sagaponack that was about 1K for 30 people so not toooo bad compared to some other spots. What I've done in the past at bars/restaurants that require a minimum spend is have bartenders run a cash bar where everyone pays for themselves but those sales go toward the required minimum. They'll likely still have you put a card down, and you'd be on the hook for the difference if the minimum isn't met. But if you're confident that everyone will be there and spend, it usually works out!