my friends are asking me to make them cakes too
Are you "legal" ? If you sell cakes, it is usually required that you be registered as a business, be insured etc etc .. You better find out before you start selling ..
To price a cake, you have to cost everything - materials, utility consumption, machinery depreciation, taxes, insurance, delivery costs, cleaning, maintence, etc etc plus the cost of your time, effort, expertise etc etc.
Pricing also takes into account rural/urban, geo-income location, competitor pricing, etc.
Cake baking/decorating is a business. You can charge per item or per slice. Undercharging a client makes the cake a gift from you not a viable economic product.