As a commercial designer in the UK, and as something of a PS guru, I charge 35 an hour. (Try to avoid at all costs doing an all-in price, as so many clients think they are designers, and they will have you chasing your tail for weeks if they know they can). As the other guys said, a lot depends on how quick you think you can do it. I am an experienced user, so I can charge a lot but do it quickly, you are maybe less experienced, so you charge less, but it takes you longer, so the final charge may be the same, except I have my feet up while you are still working!! The image size is also important, as if it is going to be high res (ie say a 150dpi poster), then this may slow down your work a lot unless you have a super-whizzy computer. Bear this in mind. Also, add at least 20% more time to the job than you think it will take, becaue people invariably underestimate, rather than over. Hope this helps. email me if you want a chat on the phone.