Events are available with all Aqua Enterprise Plans.
If you enabled Events via CloudWatch Events:
AWS will charge you for SNS endpoints ($0.50/region/million calls), and CloudWatch Events delivery ($1.00/region/million calls). 99% of users will see less than a $5 increase from AWS per month.
If you enabled Events via CloudTrail:
AWS will charge you for AWS CloudTrail to send write-level API activity to its S3 bucket for processing. AWS charges $2 per 100,000 CloudTrail events.