I recommend not giving out this information. Instead, politely tell them this is private information, just as the salaries of their employees is private. Offer them an opportunity to discuss what makes you valuable to their company. This question can be used to see if you are wasting each others time (the position pays far less than what you are used to), or to see if they can offer a low ball salary if you current/last salary was at the low end of their scale. Let them throw out an offer or number first.