What is the best way to answer?

Sites like glassdoor.com and others can give you a decent range for most job categories.

Answering that question is easier if you understand:

How much experience in that field do you have? If none, you can't command high dollars.

Do you have an education past high school?

Did you sell yourself in the interview or are you just one of many who are not standing out?

What is the total compensation package? There's more than just a salary to consider.

I usually put "open" and leave that for discussion, after I get an interview and have sold my ability. The salary talk normally comes with the job offer.