If you can't give the cake as a gift, and your friend insist on paying, then just charge for the supplies. When I am doing a cake for family and they insist on paying me (I always offer family free) I just ask them to cover the supplies but I'm not sure if this is still considered selling a cake.
It is always a good idea to check with local codes or to see if your state has a cottage law.
When it comes to pricing a cake, a lot of factors come into play. What I pay for my supplies and what you pay in the state you live in can differ. Also what other bakeries and cake decorators in your local area charge for cakes also factor in. We all live in different locations around the country and pricing also varies due to this. Big cities vs. rural locations.
All this is to say, no one person really can offer real pricing advise to someone because there are so many factors that go into it. It takes lots of research.
Good luck and don't get discouraged... it never hurts to ask questions! It's all a learning experience. I also believe if this type of question has been posted multiple times, instead of people getting tired responding to them, just don't respond/post. Move on to the next thread that interests you.