We do not recommend putting the price on the back cover.
One of the best parts of self-publishing is the ability to control your price. When the price is not included on the back cover, you are able to change the price whenever you wish, simply by logging into MyAccount and making the change. This is free to do, and takes just a few seconds (For more information on changing your retail price, click here.)
If the price is printed on your cover, you will need to pay for an update to the back cover file (reflecting the new price) in addition to the steps listed above. This will cost a minimum of $199 each time a price change is made, and there is no guarantee the updated files will be used on all future POD orders. Retailers order copies of books to keep on hand, and will not order more copies until their existing stock is sold. Depending on the demand for your book, it could be months before the new file with the updated price is used.
Even if you don't think you will ever change the price, production costs are constantly in flux. If the cost to produce your book eventually exceeds the royalty rate, you will be required to increase the price.
Simply put, while we can add the price to your back cover, we do not recommend it.