All positions within a company have a budgeted range. This question is just a way of weeding out those with too high of expectations. If you respond with a number that is low, don't expect to be able to negotiate any higher. If you respond with a number that is high, you take a chance on not getting the job. For starters, don't talk money until later in the interview process. Get all of your other questions answered before talking money. Let the interviewer know that you will be open to discussing fair compensation when the time comes. If pressed for a more specific answer, always give a range, rather than a specific number. A good answer might be, "My salary expectations are negotiable based on the total compensation package." This is stating that you need to know the value of the benefits before you determine the salary you need. Or, "There are so many things to take into account such as what the people I would be working with are like, the type of work, the benefits, and so forth that it's not something I could estimate at this point."
"Let us be thankful for the fools. But for them the rest of us could not succeed." - Mark Twain
"There is no death, only a change of worlds." - Native American Proverb