What is the approximate cost or if qualified, would legal aid be able to help expunging a past charge?

As long as you were not convicted of it, you can expunge it. As far as free legal service, I don't think that is available for expungements. You may try shopping around to get a good price, or call the Allegheny County Bar Association Lawyer Referral Service to see if they can direct you to some sort of reduced fee panel.