This document explains Google Cloud Armor pricing details.

  Google Cloud Armor Price
Per Policy Charge $5 per Google Cloud Armor security policy per month
Per Rule Charge $1 per rule per month, pro-rated to the day
Incoming Requests Charge $0.75 per million HTTP(S) requests that are evaluated by Cloud Armor

If a backend service has a Google Cloud Armor security policy associated with it, you can use the user-defined request headers feature with that backend service without any additional charge for the user-defined request headers feature.

If you pay in a currency other than USD, the prices listed in your currency on Cloud Platform SKUs apply.