This is when your preinterview research comes in handy. Take a look at salary guides for the particular position in your region. These should provide accurate ranges for you to go by. Do not name a specific number but do state that, "The research I have done shows that a person with my experience is usually paid in the X range for a job such as this one." If you are unsure of what the range is then an answer of "I care much more about getting myself the right opportunity then a certain dollar amount" should get you through this question.