How much do you charge for alterations?

I seamstress on the side (used to work in a dress shop so I have the professional skills bus work in a different field now). And i always feel weird choosing prices for my work. When I do alterations for family/friends I usually leave the cost up to them and its hit or miss on if im satisfied with what they give me. I know how much these things would cost for them to go get done at a shop and would never charge as much as a shop/company, but dont want to undervalue my work either. How have some of you found what to base your rates on? Time? Skill? Or just have a general number at the ready like $xx for sleeve hems and $xx for dress hems etc?