That varies tremendously. My best recommendation is to purchase the Passporter Disney wedding book which has everything you need to know.
To give you a basic guideline - a wedding for 6 (memories) is about $2,500. That's the lowest you can go, if you want to go through Disney (you can also look into Swan and Dolphin weddings through Just Marry! or Shades of Green). The next package is for 20 people and those weddings start at about $5k.
If you're interested in a wedding for more than 20 people, the minimum is about $12k but the sky is the limit.