For Mentimeter Pro and Basic, we do not provide monthly plans, only yearly subscriptions. For more information, please take a look at our Plans and pricing page.
If you are planning to run a one-time event, such as a conference, we do offer a one-off pricing plan. Please visit the Mentimeter for Conferences page for more information and to purchase this plan.
However, you can always use the Mentimeter free account for as long as you'd like before you decide to upgrade. Read this great article to learn how to optimize the use of your free account.