If you are a developer looking to use Authy for your project's two factor authentication, you'll find that our cost is only based on successful authentications and delivery costs. We also offer flexible pricing so costs scale with your usage.
You can see full details on our Authy Pricing page.
If you are an end user looking to add better security to your devices, Authy is free to install and use. Please learn more in the article Installing Authy apps.