Hhhmmm... My boss does graphic design stuff for a few businesses around town and he never charges less than $20 and hour, more if it's more difficult or if he has to run around and talk to a lot of people to verify measurements or find people to actually apply the designs etc.
If i'm drawing up a tattoo for someone, our shop as standard practice charges an initial $20 drawing deposit to even begin work on a drawing, $40 for really large pieces. (There's too many schmucks who'll just never come back, or show up a year later expecting you to have held on to the drawing for their hour and a half duration piece for all that time) Then we go by the hour as far as how long it's going to take to tattoo them. My rate is set at $100 an hour, but that's only for how long it will take to actually tattoo it, so while alot of people think that sounds unreasonable, they don't take into account that i might be spending three to five hours drawing it up. So for a two hour tattoo i'm charging approximately between 25 - 40 buck an hour, but i still have to share the profit with the shop after the fact, and i keep this in mind when people try and talk me down or haggle. I'll politely state my original rate once, and after that i go progressively higher each time they try to 'make a deal'.
If you know what your time is worth DON'T SELL YOURSELF SHORT! If you're really serious about the work and you're going to put the time in, they'll most likely be satisfied with the finished product and not as concerned with the initial cost as one might think. I've never once heard anyone complain about the cost of a tattoo once they were finished.
"Today, a young man on acid, realised that all matter is merely energy condensed to a slow vibration. That we are all one consciousness experiencing itself subjectively, there is no such thing as death, life is only a dream, and we are the imagination of ourselves... here's Tom with the weather." - Bill Hicks