It’s a tough question with a myriad of approaches. From an artist perspective as well as from a buyers point of view. The link below is a pretty full featured blog on pricing that’s worth the research time. Of course it doesn’t give a definitive answer as to the best approach. That is, after all, a personal choice. But it does list out the options and has some thorough research sources behind it.
Hope this helps. I would be interested in hearing your thoughts as well.
Thanks, of course to “Makingamark” at squidoo.com