Probably more than you'd like to pay. Why? This is YOUR day! You should expect more professionalism than the status quo, than what's been set out there as acceptable. Here are some things you really should be considering, instead of "just the cost!"
Is your performer in it just for the money, or is he/she in it because they truly care about the outcome of your event? Some DJ's barely charge enough to break even on the cost of labor, supplies and equipment. But the love of the job makes it worth while.
Bride & Grooms spend thousands of dollars between the reception hall and catering alone. They'll spend about a thousand on a photographer. Engagement photos, the photos at the ceremony, photos after the ceremony and then that photographer will only stay about an hour after dinner to get the "first dances" pictures. They'll even spend $350 on a vegi tray! But, honesty ask yourself this; what are the two main things you remember about a reception? The catering and the entertainment. How was the veil? Smothered chicken? Beef tips? On the other hand, when it comes to hiring a professional DJ, they don't think it's worth spending anything over $300-400. Isn't the success of your night, worth more than a vegi platter?
Here are some things to think about. Does the DJ you're considering rely on Expertise or Ego? Do they deserve your trust? Have solid MC skills? Know how to use their equipment properly? Have professional equipment for that matter? Understand the pacing and timing of the reception? Have the personality that reflects what you're trying to achieve for your reception? Care about your wedding? Are they honored to be apart of your big day, or is it just another pay check to them?
Professionalism is defined as competence and skill. The professional understands and cares that your day is special and will use his competence and skills to help make your day enjoyable and memorable. For the stereotypical DJ's, the common perception is that they treat weddings as just another "gig" on the calendar.
So, how much should your Wedding DJ cost? There's no exact science to give you that answer. A lot of factors should be considered; like your visions, needs, wants and even budget. Here's a guideline to what you should expect to pay for... How important is the success of your reception to you and your fiance? Do you want classy intros, someone who entertains instead of interrupts? A system that won't look tacky in photos and great music & dancing at the reception? Someone to give you advice and guide you on a timeline for toasts, cutting the cake and first dances? Someone you can trust? That will give you peace of mind that everything will go smoothly and positively impact the success of your wedding reception?
If you've said yes to any of these questions, you want to hire the best, most Professional DJ you can afford. What do good DJ's charge? I'm sure you'll find cheaper DJ's than what I charge (for I've lost many potential clients to lower cost DJ's), but if you want a really great DJ, expect to pay more than some of the estimates you'll receive from other companies.
I hope this helps in your search for quality entertainment!