A tricky question. Do not answer it immediately. Instead, say something like, That’s a tough question. Can you tell me the range for this position?
In most cases, the interviewer, taken off guard, will tell you. If not, say that it can depend on the details of the job. Then give a wide range.
Note that whenever you are going for an interview, this question may be asked. Before going, try to find out what the average salary for someone holding that position in that industry is paid. This would help prepare you for what is in front of you.
Do not forget that this is only an interview and you haven’t been offered the job, so do not go on negotiating. Just state something within the range you have researched and moved on. Whatever you do, do not sell yourself short.