RDS Enhanced Monitoring

see bottom of this page https://aws.amazon.com/rds/faqs/
Copied and pasted here for convenience :

Q: What impact does Enhanced Monitoring have on my monthly bills?

Since the metrics are ingested into CloudWatch Logs, your charges will be based on CloudWatch Logs data transfer and storage rates once you exceed CloudWatch Logs free tier. Pricing details can be found here. The amount of information transferred for an RDS instance is directly proportional to the defined granularity for the Enhanced Monitoring feature. Administrators can set different granularities for different instances in their accounts to manage costs.

The approximate volume of data ingested into CloudWatch Logs by Enhanced Monitoring for an instance is as shown below:

Granularity 60 seconds 30 seconds 15 seconds 10 seconds 5 seconds 1 second
Data ingested in CloudWatch Logs* (GB per month)        
             0.27       0.53       1.07      1.61      3.21      16.07

Edited by: awskyleh on Feb 6, 2019 4:27 PM

Enhanced Monitoring has no cost itself, but it does write to Cloudwatch. So when/if you exceed the free tier for Cloudwatch Logs (5GB) you'll start to pay for the extra usage. You can limit the amount of data being sent to Cloudwatch by using coarser granularity monitoring. Cloudwatch pricing is at https://aws.amazon.com/cloudwatch/pricing/