Guardduty on AWS organization vs individual accounts

To answer the questions in reverse order for you, there are no additional AWS cost implications by managing Amazon GuardDuty at the AWS Organizations level. By managing it at the AWS Organizations level however, you can consolidate findings into a central view and manage account enrollment centrally, thereby saving time and effort by not having to log into each account individually or do additional work to export results from each account.

Technically you still do enable GuardDuty in each individual account, but doing it at the Organization level makes the task centralized and efficient.