Nice idea - provided you offer the same deal to the following :-
The person you bought the flyers off.
The person who hires you the venue - who in turn offers the same deal to the bar staff & the following days cleaner.
The door staff (a brave person to try that !!!)
The person who hires you the sound system.
A DJ is part of the whole package of the night - pay them the fee agreed (as with all of the people above).
If you can't afford it - don't promote !!!!