The price of your book is mainly made of three components: the cost of producing the book, the discount demanded by the retailer and the commission you wish to earn off each book, called the Author’s Earnings.
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Since the cost of production is fixed depending upon the number of pages and dimensions of the book, and since most retailers generally demand a discount of 50% or upwards of that, the author’s earnings needs to be fixed appropriately so as to maintain a balance between earnings and sales. Lesser earnings imply a lower price for your book and might translate into larger sales. If the book sells well, you can always increase this component incrementally on later editions and earn more from each sale.