Meetup is key for discoverability. Both because Meetup is where people actively go to look for something, and because they may incidentally see your group/event while doing something else on Meetup. There's nothing else that gives a group local findability quite like Meetup. That's what you're really paying for, although agreed that the price is an obstacle.

Also agreed that migrating to Facebook usually doesn't work. In any group, there's a quarter or third or so who refuse to use FB, and that tends to kill your critical mass.