Well I've been in the "business" for over 20 years and still always under price myself. It's hard to say what to charge. Not knowing the extent of work you have to do and what these clients have to spend. I've found a good rule of thumb is to set an hourly rate on something like this. I useually start at $35.00 US dollars an hour, and up to about $50.00 an hour. But some clients like to just set up a flat fee and go by that. Also concider that these are book covers, and they will print up a number of them. So in commercial art this may fall into a "Ussage" type of price structure which is generally a bit higher than something that's used for a "one time only" printing. I.E. Book versus a magazine ad. I know this may be a bit confusing and I know I didn't give you too much to go on, but I hope this helps at least in a small way. James