As a professional photographer, it's largely down to you to decide how much your photography, and therefore your time, is worth. The price for prints you're selling will be influenced by the amount of time it takes to capture the photo and print out the product, and by the cost and quality of the materials you use. While there's no set price you should be charging for a photo, there are a number of things you can consider. Before setting your prices, you should understand your customers and charge an amount they'll be willing and able to pay. Plus, you should see what your competition is charging and calculate profit margins to actually make money.