Are you asking as user or publisher? The extra cost depends mainly on how much you utilize the running Actor and the idle time settings. But standby doesn't allow PPR/PPE pricing yet (this is in progress)
As a user. I'm using smsapi, and would like to have a standby actor to process sms when there're any (they would be sent via POST requests to the actor) and decide to whom it will forward (based on some internal rules)