It sounds like the right question to ask, but you might want to consider asking a few more questions first...
As a professional DJ for nearly 15 years I have been asked that question the most. Rather than using that question as your primary filter consider meeting your DJ in-person. If a DJ is to adequately assess your needs they need time to meet you and go over the details of your wedding. To only or even primarily go by price in choosing your wedding entertainment may lead to disappointing results.
A DJ should be interested in YOU, and not just the date you have in mind. They want to know what you are really looking for in a wedding ceremony/reception, while helping you to achieve that. Save yourself some heartache and use this forum to find out about the more probing questions such as...
1) Do you stay up on current wedding trends?
2) What wedding resources (books, music, magazines) do you recommend?
3) Are you a member of any professional organizations (ADJA, BBB, NACE)?
4) Are you licensed and insured?
These are just a few questions to get you started. I look forward to hearing from you and helping you to ask the important questions in hiring your wedding entertainment.