One of the hardest parts of starting to sell art is pricing it. It is hard to put a price tag on your time and creativity. You can keep track of the supplies you use and then add extra For your time for handmade items. For paintings, a simple method of calculating the price is to get the square inches of the piece by multiplying the height by width of the painting. For example a 10 by 10 inch painting is 100 square inches. Take the square inches and multiply that by $1 – $6 depending on your skill level/reputation and what you are comfortable with. Then take the cost of the canvas or paper and frame and you have the base price. For gallery or fine art websites you want to add the commission of 40% to 70% depending on what you will be charges.
For Photography, you need to figure out cost of the prints along with money invested to capture the image like model fees, travel fees and so on. Decide edition size or single original image. Also keep in mind camera and equipment usage/depreciation.
For the emerging artist, you will need to do research in your medium what another artist are charging and making. Not just what the art is priced at but what the art has sold for. Also consider what your goal is, cover cost of making art with extra money to do more projects or to be a full time artist.