What about a restuarant? If you know of a good one in your area, that would sit about 120, some will give you a good deal on exclusive use.
A picnic/summery theme can be inexpensive. Sandwiches, tea cakes, jugs of pimms etc. Even cheaper is you could use a public park or know someone with a large garden. If its not public property, you can buy wine/spirits yourself from a supermarket, & have someone act as barman.
Know anyone with a barn or large garden shed? They can be decorated in so many ways & can look fabulous.
Having a winter wedding or having a weekday or Sunday wedding can also cut costs.
Ebay & charity shops are great for centrepiece vases, tea lights & holders & jars for favours. Any home-made favours can save costs. Chocolates, infused oils like rosemary, garlic or chilli, home-made sweets like mashmallows, shortbread, fudge are always appreciated.