There are actually several things you should consider before selecting an entertainer:
Spending more money will not guarantee you the best entertainment for you particular affair. You will have to select the DJ that's right for you, and your guests.
Is the entertainer experienced in providing these services for your event? How much experience does he or she have? Get references from people you trust.
Is your entertainer insured? Every legitimate business has insurance.
The type of equipment the DJ will be using at your event? There are many DJ's that get by, using beat-up cheap equipment because they can't afford quality. This will impact the quality of sound you will get when your music is being played.
Lighting is another option to consider. Some DJ's charge extra for it, some offer none at all, and some might have very elaborate packages to choose from.
Should something go wrong, is back-up equipment immediately available? Should a problem arise, and you are relying on the entertainment, don't you want to know what the entertainer's backup plan is for your very important event?
What happens if your DJ cannot make it? You should have a contract that answers this question.
Who will be the entertainer at your event? Many DJ companies have employees under them that you have never met before? Some of these might even be beginners! Your contract should make this clear as well.
The all important references! Ask for them and actually check them! Satisfied customers say "quality" best!
What other services are included other than just playing music? Do you get an emcee, a coordinator, how about someone to interact with your crowd, to inspire them and encourage them to participate?
Will your DJ take requests, or tailor the event to your specifications. Will your DJ spend time with you, to find out what you want and help you plan your event?
Will your DJ speak professionally in front of your guests?
Are there any hidden or extra charges for expected services, travel expenses, or requirements from the entertainer that seem unusual? Do they take smoke breaks, drink alcohol or wear a baseball cap on the job? If they don't cover these topics, perhaps you should.
In the end, you must choose who you feel is right for your event. Keep in mind the DJ you select, will dramatically impact your event, your memories and your reputation. Invest some time on your decision, and you will get favorable results.