according to the documentation there are three kind of audit: Admin Activity audit logs, System Event logs and Data Access audit logs.
which kind of logs GKE cluster is being generated? its not clear for me. i would like to understand what will be the pricing
As per documentation:
Admin activity logs - those are enabled by default at no cost
logName="projects/[PROJECT_ID]/logs/cloudaudit.googleapis.com%2Factivity"
protoPayload.serviceName="k8s.io"
logName="projects/[PROJECT_ID]/logs/cloudaudit.googleapis.com%2Factivity"
protoPayload.serviceName="gke_cluster"
Data access logs - Data Access audit logs contain API calls that read the configuration or metadata of resources, as well as user-driven API calls that create, modify, or read user-provided resource data. Data Access audit logs do not record the data-access operations on resources that are publicly shared (available to All Users or All Authenticated Users) or that can be accessed without logging into Google Cloud. Data access logs are something that can log very extensively depending on the configuration. Those you need to enable yourself, may be billed according to Quotas.
There are 3 kinds of operations, you choose which you want to log:
resource.type="k8s_cluster"
logName="projects/my-project/logs/cloudaudit.googleapis.com%2Fdata_access"
For most projects Admin activity logs is enough. If you need to log what type of resources which service / user access then you should enable Data Access logs.
More here GKE Audit logs