As a DJ I often encounter this question. It’s hard to see a higher price for a wedding than party when you are looking to hire a DJ. The reality is that a wedding takes a lot more work than a party. When you are looking for music for a wedding the needs are more complicated. If you are playing during the cocktail hour, dinner, and dancing there are several different needs for the same event. Most of the songs will need to be selected in advance and flow into the overall experience to create the music that shapes the sentiment of the event. This is why wedding are often booked three to five months in advance. Additionally, weddings involve more contact with the customers than other events. Usually a DJ will meet or call a couple several times to coordinate the details and the level of services required. Usually I spend 8-16 hours a week for the two months leading up to the wedding to create a range of musical possibilities. While I will not have the time to play all the sets I create, it gives me flexibility to respond to the needs of the audience and work in the set music that has been request. There is also a lot of pressure at a wedding for everything to go perfectly---you want to create a once in a lifetime experience for the couple and this takes time, effort, and talent.
Parties also require planning, but tend to be less pressure and more flexible. Generally parties are booked a month in advance and while there is a set preference for types of music to play and specific songs requested, there is a lot more freedom in the music genres and styles being played. While parties also require planning, they tend to be less formal and one of the most important qualities in a DJ is responding to the crowd. Each party has its own personality and your DJ will need to account for this. Sometimes your budget will determine what you can do. In these cases talk to the DJ you hope to hire. See if you can negotiate a price that will suit both your needs and that of the DJ you are hiring. A good DJ will know the limits of what they can do and may be able to offer suggestions and/or recommendations.