You definitely should call the Disney Wedding planning people. There are a lot of less expensive options for weddings at the park.
There are, but the curve ball the OP threw was the 100 guests. The more affordable "Escape" options are capped at 20 people, but are ~2k to 5k. So if they are able to cut down their guest list, then they will most certainly be able to do it for much less money.
* Conch Key Marina at Disney's Grand Floridian Resort & Spa $1,991.00
* Narcoossees' Landing at Disney's Grand Floridian Resort & Spa $1,991.00
* Luau Beach at Disney's Polynesian Resort $1,991.00
* Sea Breeze Point at Disney's BoardWalk Resort $4,750.00
* The Wedding Gazebo at Disney's Yacht Club Resort $4,750.00
* Sunrise Terrace at Disney's Wilderness Lodge $4,750.00
* Sunset Pointe at Disney's Polynesian Resort $4,750.00
* Disney's Wedding Pavilion $5,750.00
* Canada Pavilion at Epcot® $5,800.00
Those prices include
Floral Bouquet & Boutonniere
* Fresh floral bouquet
* Fresh floral boutonniere
Cake and Champagne
Wedding Cake and Champagne
* Beautiful two-tiered wedding cake
* One bottle of our signature Fairy Tale Cuvee (pours approximately six servings) or two bottles of non-alcoholic sparkling cider
* Keepsake Mickey and Minnie cake topper
Entertainment
Entertainment
* A violinist to play during your event
* In Magical Ceremony Locations, a trained organist will accompany a violinist for your walk down the aisle.
Limousine
Limousine
* A limousine charter for four consecutive hours
Photographer
Professional Photography
* A professional Disney photographer to capture your event
* An album of 20 4x6 prints
Wedding Websites
Web Site
* A personal Wedding Web site
Disney Wedding Coordinator
* A Disney Wedding Specialist will coordinate your wedding and ensure that your events runs smoothly.
Annual Passes
* The happy couple will enjoy Complimentary Annual Passes to Walt Disney World.
* Annual Passes become active from date of first use.
You may need to do things like buy more champagne if you want it, but it is still much cheaper than the Wishes collection. The number of guests per location varies, some max at 8, others can do up to 20.
That really should give you an idea of what the ballpark prices would be, but definitely call DFTW to discuss details with them.