I've seen salary expectations on application forms, but I have never been explicitly asked in a pre-interview email what my salary requirements are. I am applying for an entry level software position in the greater Boston area, so salaries certainly vary. However, I don't want to cut myself short by asking for too little, or ask for too much and not even get the interview. How do I respond to the question?
This is common (and not the same as a company asking what salary you'd want), as there's no point a recruiter spending time looking at salaries for $20k if you're only interested in, say, $35k as an absolute minimum. (Figures completely pulled out the air.)
Do the research, decide what you're worth given a few positions you might find, and then just give the recruiter a range that you're happy to work with.
I think it's reasonable for a recruiter to ask what your salary expectations are. If they are recruiting for a particular company, they can avoid presenting candidates who are outside of the company's budget. If they are recruiting for many roles, they can put you in for roles that meet your salary needs. Both of these assume that you are asking for a reasonable rate.
As far as trying to figure out what to ask for - do research. Look at various postings and use services like LinkedIn and GlassDoor to find what companies are paying people with similar backgrounds. Be sure to consider geographic area and industry. Give a good range (not a single dollar value) based on what you find. You may also want to consider mentioning benefits that are very important to you, if any, especially if you are willing to work for a little less cash in favor of some kind of benefit.