There's more to a professional wedding DJ than the time spent behind the booth. Here's what goes into the cost.
Vehicle cost, maintenance, insurance, and gas (especially to remote venues and client meetings) all play a role in the cost of the event. Purchasing hundreds of songs each year (that's a lot of Justin Bieber) adds up as well.
DJs normally bring thousands of dollars of professional sound and lighting equipment, cables, microphones, and stands to an event. These are periodically replaced to keep them top of the line! If a couple were to rent this gear on their own, they could expect to pay hundreds of dollars (setup not included)!
It costs to maintain a professional website, along with making sure new clients are aware of a DJ's presence through flyers, social media, and other means.
A good DJ will spend time to sit down with a couple at least once to plan their special day, and then spend at least an hour or more preparing music, gear, and timelines. Including prep, packing and loading, travel, setup, reception time, tear down, and travel home, a DJ will typically spend over 12 hours on your special day.
You can't really put a price on this one. It's what separates the professional from the hobbyist. Number and variety of events performed, years in the industry, reviews from past couples, and photos/videos of events are good indicators of a DJ's experience. A quality DJ is like full coverage insurance. It gives you peace of mind on your special day.