think of unique places to get married - museums, backyards, parks. chances are, if it's a random place, they haven't dealt with many weddings. that means they probably won't be as organized, but it also means they'll be cheaper.
The more unique, the cheaper it could be! The zoo in my city will host and cater a wedding for 200 people for around $1500, which includes tent, chairs, food, servers, etc. Tent and chair rentals can end up being as expensive as renting a hall.
Also, having the wedding on an 'off' day or early in the day, as others have suggested, will also reduce the number of people who accept the invitation. That makes it easier to invite people you might not necessarily want to invite, knowing that there's a good possibility they will not accept. (but then again, they might)