For any business you need to know your cost of goods sold and your fixed costs
Your cost of goods sold are anything that are per unit such as purchase costs, shipping per unit, packaging etc.
Fixed costs are a regular cost to your business for example rent, wages, etc. these need to be estimated unless you know exactly how many sales you will make per month, year and divided by your expected sales.
Usually businesses work on a good guessimate as to cost of goods sold as things like multiple orders can reduce any shipping costs and your fixed costs will be divided by your expected sales and added to cost of goods sold figure..
Once you have a realistic idea of your costs then many companies work on a multple of the cost of goods sold or a percentage as the sales price.
Just list all the costs associated with the product and put them into either cost of goods sold or to give you your Gross profit then make the calculation to calculate your expected net profit.
A spreadsheet is great for this as it can help you calculate your ideal or required profit.
A number of free business plan software also have this facility and more.