The Monitoring Service is an add-on which is available in all plans and billed independently from them. E.g. you may have the Free Plan and enable monitoring in the servers and sites of your choice: You'll only be billed for the monitoring of such servers.
The cost of the monitoring service is $5 USD per server per month (at most). The service is billed hourly at $0.006944 USD per hour. Taxes (if applicable) not included.
Monitoring a server allows you to monitor the sites it hosts without additional charges.
During your current billing cycle you can check your monitoring usage at any moment.