The best way to price art is to pay by the hour/minute, like a real job! Because art is a real job (Don't let any tell you otherwise!) So, lets say your base rate is eight dollars, and it takes you two hours to do a picture, charge sixteen dollars! Eight dollars is a good base rate, because it's considered the average "minimum wage", though you can change it to fit your circumstances.

Do you have bills to pay and the minimum wage in your are ten dollars? Charge ten an hour!

Just want some spending money? You can charge less (or more!)

I guess what I'm getting at is charge what you need to in order to fit your needs. This is advice from to artist to another.