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A less expensive banquet hall are community centers or rental spaces in hotels/restaurants. They offer the same space though you may have to work out rentals for china/cutlery etc. Check out parks if that is an option for you based on the guest count.

You should plan on finding a space that holds at least 300 in case Ontario's covid gathering limits are still in place.

There are local community centres or churches with party rooms that can probably accommodate 150 standing in a cocktail style, but for seated you'll likely need something like a large gymnasium. It won't be a pretty space typical of a wedding like a hotel or banquet hall.

Any other event space that can hold 150, especially those that deal with weddings regularly, will not be cheap especially during August when it's an in-demand season for such events.