While licensing is out of the scope for this book, we feel it is important for developers to understand the rough edges and limitations of a given service in order to be able to build performant and robust cloud solutions.
Office 365 licensing is a fixed per-user license that is billed monthly (or yearly if you choose to pre-pay for a full year). You can have one or more different types of licenses, but only one license type at any given time for a specific user. You can, of course, purchase a higher tier license, assign it to a user, and later change that to a lower tier license when the needs of the user change.
When writing about public cloud services such as Office 365, it is an assumption nowadays that prices fluctuate. You can always verify the current prices at http://www.office365.com. Make sure to change your country at the bottom of the page to get pricing in your preferred currency.