I use one of two responses depending on where in the interview process I am.

1) Early in the interview process I respond with: "I’m sure we’ll find a number that works for both of us, but for now, I want to make sure it’s a good fit."[1]

2) If it is during a final round or they are offering the job, I like to turn the question on the interviewer and ask: "What is the salary range you have in mind for this position?"

1. http://bucks.blogs.nytimes.com/2010/06/17/on-refusing-to-dis...