First, you always want to make a profit. Not necessarily a huge profit, because readers get annoyed at feeling taken advantage of, but at least some sort of a profit. You shouldn't lose money selling your books. So whenever possible buy the books through your "back door" method at your publisher. Nearly every publisher offers a discount to the author for buying their own books. With CreateSpace and Amazon, you want to log into the CreateSpace author interface and buy your books there. You then get the bare-bones author price rather than the retail price.
Next, you have to account for the shipping of those books to you and the sales tax Amazon (or whoever) charges you for the sale. Usually you buy in bulk to save on shipping costs so that's a factor as well. All of that becomes your initial base cost. Divide that by the number of books you bought together, and now you have the starting price per book.
Now look at the retail price for the book. That gives you the other end of the range. Generally you want to sell your books for less than that retail to give your readers incentive to buy from you. After all, normally the retail price contains a big chunk of money going to the publisher (Amazon or whoever). Since it's all now going to you, you make more anyway.
Make sure you include the sales tax that YOU as the seller must pay the state you're in for this sale. Nearly every state in the US (and every country outside the US) charges a sales tax per sale. So figure out what the sales tax is for the location you're in and factor that into the price. Make sure you have the proper sales tax license for selling, too. That normally isn't something the venue can "cover" for you. You're the vendor. You need the license to sell.
So what you end up with is a spreadsheet like this. List the quantity of the book you ordered and its title. List the total price it cost to get that book into your hands. Divide that by the quantity of books you ordered and you get the per-book price.
Then look up the list price for your books and determine the price you want to sell at, somewhere between your base price and the total cost. Calculate out the sales tax for that sale. Subtract that sales tax from your selling price and you now have the profit you'll actually take home for each book sale.
Note on this example spreadsheet there are a few of my Low Carb books that I bought retail from Amazon. Therefore I have to sell them for slightly ABOVE retail in order to make a profit. Normally I wouldn't recommend doing this, of course. In this particular case I wasn't intending to sell the books at the Boston Book Festival, but I wanted to bring them along for show-and-tell. Still, if someone really wanted to buy one, I wanted to have a price to tell them to cover my own expenses.
So this is stage one. This is for your own private notes. It isn't something you'd show the buyer.
From that information you construct the public price sheet.
It's good to have the price sheet for various reasons, even if you put price stickers ON the books. Sometimes those price stickers fall off. Sometimes you have a friend helping you and they want the sheet to quickly refer to. Always keep a price sheet on hand. And put a copy on your phone, too, so if the sheet gets lost you have something to refer to.
Make sure you keep track of how many you sell of each book, often by putting a tick on that price sheet, so that you can then properly pay your sales tax to the government.
Good luck with your sales!
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