I think Desnik will be using a Disney DJ. That price would be right... otherwise an outside DJ would only be charging less than $100. $900 to $950 is the correct range that they will charge for using a non-Disney approved DJ.
I personally have played ALL time frames: 3 hours, 3.5, 4, 4.5, 5.. etc etc. Disney will simply charge you for the hours that you want the DJ to play. Remember, too, a DJ will also not take any breaks during the evening, you will have continuous music throughout.
Hope this helps!