I charge like this, the size of canvas I use, the time I spend on it, and the materials I use. Factor in your costs, and then double that price for your selling price. That is what I do, if it makes sense. Remember its a job, and its a business, you want to make money off it.
Don't undersell your work or you will be out money. Art is expensive because of the hard work that an artist puts into it, its not just an image someone buys they are paying the artist for their hard work and their time. I hope this makes sense.