I wont copy /paste the complete post, it is kind of lengthy but worth the read. it really does a good job talking about the differences between a professional DJ and the hobbyist and why there is a difference in prices. This article is one of many that are slowly popping up as more cheap and bad Djs flood the industry because technology makes it too easy and there is no real accountability.

http://theflirtyguide.blogspot.com/2010/06/how-much-should-your-wedding-dj-cost.html