Pricing is possibly one of the most changeling things we must do as professional artists. I can't tell you how many times a potential client has insulted me with, "Why is it SO expensive!?" The client rarely takes into account that as an artist you are providing them with a specific service and that we don't just do it "for fun." We have bills to pay just like you. It's doubtful that someone would challenge their accountant, doctor, or plumber in the same fashion. Typically they expect those services to cost a certain amount so they are willing to pay the fee. For artists though, it can be tricky- especially with places like Deviant Art where you can find talented young high school students willing to work for free!
I often get asked how to charge for artwork, and I have a much more "wordy" and in depth answer to that in my FAQ section here, http://www.artnuvogue.com/2013/05/faqs.html#artbiz
But recently I stumbled upon this fantastic little formula by David Airey (http://www.davidairey.com/) a professional graphic designer from Northern Ireland. He simplifies the process so perfectly here! While obviously design pricing isn't an exact science- these are definitely the big points to consider when charging for your artwork! Hope this helps you along your journey toward being a pro! xoxo L