A Question on Charging for Artistic Services...

I figured you all might be able to help me on this...

A friend of mine has asked me to do a business card design and some fliers for his new business. I've been keeping track of my hours but there's one thing that I'm puzzled about... how does one decide a fair rate to charge for service? I don't want to charge to little for my services and cheat myself, but I also don't want to cheat him by charging too much. So, how exactly does one come up with a fair rate?

Any help would be much appreciated...