I’m not a graphic designer but have hired several before. From my experience in web design, professionals like designers usually charge in line with their experience and talent. If you are talented and have a decent portfolio, you can charge more than if you don’t have those things (even if you are just as good as someone who does). Most of the designers that I have hired that have these credentials charged in the 30-40 dollar per hour range and some did work on a per-project basis.
Here is some info on average rates according to the department of labor,
Hope that helps,