You can have a cheap wedding for 200 people but a cheap wedding for 200 still won't be 'free'.
Personally, I'd just have your parents and siblings at the registry so there's no hanging about for most folk, then straight to the hall.
But you still have to cover:
Clothes
Hall hire
Decorations (I'm pretty minimal but you need to have something to make it look nice for your guests
Booze
And food.
You can do a bring a plate wedding of course.
The 'cheapest' wedding I went to was on a campsite, where the bride and groom paid for the site and the venue. They provided some booze, but people brought their own, and they had some snacks and a pizza van turned up for two hours to feed everyone. People pitched in with decorating, a friend made the cake, etc etc.
But they still had to find clothes, decorations, venue hire, some booze, transport, the celebrant, PA hire, they wanted hen and stag doos, invites, thank you cards, etc etc etc. It all adds up.