You need to have thought through very carefully in your own mind both what salary package you can reasonably expect and also what the minimum is that you would be prepared to accept, assuming the job itself was suitably attractive. These are issues only you can decide but it will certainly help to have an awareness of what your 'market value' really is. This will take a little research. But that is not to say you should give a precise answer. Unless you have a firm job offer in hand, it is best to dodge the question slightly and quote a range of possibilities.