Dear Polly13
A few general rules need to be understood and have been used by many of my crafting friends and myself over the years;
1) You have to love doing what you are doing - be it selling and/or making a product.
2) You need to understand that at first you will not get paid for the time you invested in making one item. With time you will be able to, as you perfect your system/time in making an item. If the work is quality - customers are intelligent and know what they like. With time and as you start to do custom orders these customers know the value of your time and will have no problem paying you for that time.
3) To start out - take all the materials it takes to make one item multiply that figure by 3 - that's the formula I have used for years with lots of success.
I also round things up to the nearest dollar. It does make life a bit easier to start with. Once again with time you will decide what works best for you.
Many shows require it and I am a licensed business and account for my taxes to local and state agencies. But I have always rounded my price tags up to the nearest dollar and back the taxes out when I go to pay them. This makes the customer very happy. Dealing with change at craftshows can make or break a purchase, this I know for a fact. I've have seen customers that were sincerely interested in an item say No because they didn't want or have the money to pay for the item once it's taxed. In some cases, they have actually expressed their thoughts on not having to pay taxes at a craftshow. Some will never see us as legitimate businesses but for the most part, many of us are.
Hope that helps.