Initially Google Cloud Platform (GCP) gives you a $300 free credit buffer specifically for maps API usage. When the free trial expires, google gives $200 monthly free credit instead. These credits will be used before GCP starts billing you for API usage.
Please take note that you would need to update and validate your billing details in GCP to continue using its API services.
For API usage rate it is 1,000 search requests = $2.00. Meaning you would need to search more than 100,000 times in a month to get billed by GCP. And even then it wouldn't be for more than a couple of dollars.
GCP has this feature to avoid search request abuse, mostly done by people using bots to hoard leads.
Here's the link for your reference.
https://cloud.google.com/maps-platform/pricing