Guys!! I have just come upon an extremely useful tool for those who are trying to figure out how much to sell their work by, either for commission or when selling finished work.
I must first say that this is just one way to go about pricing your work, but I hope that may be helpful to someone!
An online art marketing and job seeking site called The Creative Group offers a salary calculator which allows you to specify the type of work you do and a zipcode. Upon putting in this information, it will give you a range and an average yearly salary for that category of professional work in that particular area. You can then compare it against nearby cities/towns and against the national average, and just go hog wild seeing how much most other artists get for their work. Keep in mind this is all in US dollars, so conversion to other forms of currency will have to be done should you not be in the US.
The calculator can be found here: http://www.roberthalf.com/creativegroup/s