Typically, you do get what you pay for but that is not always true with disc jockeys.  We often tell people that expensive DJs are not always good and cheap ones are not always bad. You really need to talk to the DJ and decide for yourself.  First, ask about the DJ's experience for your type of event.  You might also want to ask about the equipment they use and if back-up equipment is brought to the show. After all, even professional gear can break and leave you with an MP3 player and a boom box!  

Why are weddings more expensive?  They take more planning and, of course, supply and demand come into play.  Having a wedding in the middle of winter should be a little cheaper then May, June, September or October.

There are other things that might add to the cost.  A 2nd sound system, extended entertainment time as well as the infrastructure of the facility.  If your event is held at a venue that doesn't have ramps, paved access, etc. or has stairs, then cost increases due to the simple fact that additional labor is needed to load and unload equipment.  Venues that have easy access such as a loading dock, loading ramps or even elevators will allow a single DJ to load and unload.