I do pretty much as mdt does....with a few variations. If it's mural work, I charge 50 -70 bucks a square foot depending on complexity and size.  If it's digital portaits of children or pets, I might do 20 bucks a pop. (without framing or anything like that)  My standard hourly rate is 50 bucks an hour.

I based my digital work on the local printing and digital media businesses in the area. Scope them out and look at their prices. Some charge 25 or 30 bucks just to change file formats. Some charge the same for just putting it on a cd.  Many times if I list that kind of stuff for a client and tell them I'll throw all that kind of digital manipulation in for free, they get all smiley.