A tool that I’ve found is really handy is a called the Cash Clock. It’s a simple program that measures both the time that you’re working on a piece as well as how much money you should be earning. You can adjust the hourly wage to whatever you feel is right. Simply start the clock whenever you begin working on a project right up until you’re finished. It can give you a clear indication of what you should charge for commissions.
No artist should make below minimum wage for their artwork.
The Cash Clock is found as a download here.