When setting your book up for distribution, you would provide an available discount that a bookshop/retailer can take for your sale.
An example of £20 RRP and 30% discount;
When a sale is made with by a retailer, they could take up to 30% from the sale, so they would take a maximum of £6 - we would then take our production cost and the rest would be paid out in royalties
The minimum discount that would be needed is 20% to list the book on our store The Great British Book Shop.
Other retailers and bookshops would vary between 30%-65%