A friend had a wedding on his dime. But he did it in an clever way. They rented a paddle wheel boat on the Napa River for the reception. It was unique and fun, and they had a lot of Coast Guard people in attendance who like boats anyway. Also, instead of hiring a photographer, they had disposable cameras on every table. Although all the pictures were taken by amateurs, by sheer quantity they got plenty of good shots. It made for a very memorable day.
Another person I knew got married at the Santa Barbara Courthouse. That's very low budget, but very beautiful. Then the reception was at a parent's house.
So there's some very classy places to tie the knot. Be innovative, make it memorable and save your money for important stuff later.
Get as many of your friends and family to help as much as possible. And keep it simple. It's easy to say you want it simple and before you know it, it isn't simple anymore.
Pick a location that is naturally beautiful so then you don't have to spend a lot on flowers and decorating. Have a buffet type reception (cheaper than individual service).
Hire a photographer for just the formal pictures and do disposable cameras for the reception like Nic7320 suggested.
I think that the best way to cut cost is giving invitation letters or cards to the first hundred people dear to u who you would like to invite. Am make it simple but beautiful.
Have a small wedding at a beautiful park.
You can get married in December when the church is already decorated with flowers, bows, wreaths, trees for Christmas.
I don't know how you can have a wedding that is both cheap and beautiful at the same time, especially for 100 guests. I guess it would all hinge on what you consider cheap.
For cheap food at a reception you could order a couple of hundred 59 cent cheeseburgers from Mc Donalds.