At one event I organized, we sold the tickets ourselves (through Eventbrite) and we kept the ticket money. The nightclub venue hire was "free", but we had to guarantee a minimum amount of drink sales for the night, I think $2000. We reached the target, but if we didn't, we were under contract to pay the remainder in a very short time frame.

Basically we got to keep door sales if we guaranteed the venue a crowd.

Eventbrite lets you set aside free tickets, eg if you have VIPs / sponsors who should get free entry, which would make it look like you've "sold" more tickets than you really have.