Hey there Sakke,
I’ve got a friend who is a graphic designer and he charges $50/hr. Take into consideration that he is a lead designer for a well known local sports magazine. His work is flawless and he always pleases the client. So to answer your question I think you need to evaluate your experience as well as what’s worth your time. Also I would take into consideration how much time it’s going to take you. If it’s something that you can whip out in a couple of hours, you might want to charge more to make it worth your while. Oh and most importantly don’t underestimate your self.
DS