As with a normal booking, the price of an extended booking is made up of the rental costs plus the insurance premium and any optional extras.
In the case of an extension, insurance premium is calculated again as this premium is geared to the length of the rental period. If the number of rental days becomes longer, the premium will also become a bit higher. However, the extension days are charged against the (usually lower) 'extra daily rate', so you'll benefit from that.
Good to know: Extending is only possible if you click on the 'extend' button in the detail screen of the current booking which is visible during the rental period.