Job posting asking for a 'ball-park' salary requirement?

I have seen a couple job postings that ask for candidates to submit salary expectation and I am never sure how to answer. Usually the question is part of a form and not mandatory. In this case I skip over them or put a general number from other job postings I apply to.

The job I am currently applying to is asking for candidates to submit cover letter, resume, and 'ball-park salary requirements' - no indication on their end what the pay would be. Should I be including this in my cover letter then? How big of a range should I use? For example, a job I just applied for gave a range of $31 to $35 hourly. I am new to the field, so I put my estimates on the lower end - but I am never sure how to approach this.

Any advice would be appreciated!

usa jobs
usa jobs resume
usa hotel jobs
usajobs
usa jobs federal government
usa job in ksa
usa jobs
usa jobs login
usa jobs gov
usajobs.gov
www.usajobs.gov
usajobs.com
usajobs
usajobs.gov official