Subscription plans indicate the very minimum billing amount. Users can absolutely monitor more. Pricing is calculated on an hourly basis and is prorated based on the amount of actual overage hours monitored. For example, the cost for an extra resource in Professional subscription is $15/mo. This translates to 0.017$ per an individual resource-hour. Actual monthly bill is calculated by adding the base price of the plan, and any additional overage hours multiplied by the hourly resource rate.
Volume-license plans have lower hourly rate but higher minimum commit.
Feel free to reach out to firstname.lastname@example.org with more specific questions