I love Meetups as a lead magnet for business owners.
Though 80 doesn't seem like a lot, being that it's a specific niche like "Brand and Marketing Managers" its awesome.
Charging $5/$10 isn't a lot and would monetize, but its really not going to change your life either.
Does having these 80 a month help your business currently?
Sponsors are great and because of the Niche you have it wouldn't be hard to monetize that way. Check out "The Phat Startup" in NY and watch how they run there meetings.
You could pull together the 80 a month and pick a few each month to interview. Put that behind a firewall to charge and you can also create an Amazon book Series playing on the fact you've interviewed 700+ Top Brand & Marketing Leaders.
Lastly if you send a newsletter to the group you being that its so targeted (NYC) you can charge Advertisers looking to hire. Simply go to any recruiter sites see who they're advertising for and contact them. They're typically paying a lot for these services and now you've got a targeted list.