Is clustering amazon RDS multi-az replicas to reduce costs possible?

Question

We are considering moving our databases from heroku to Amazon RDS. Main reasons are costs and downtime. Heroku requires separate instance for every db and downtime is huge (60 mins a month for Standard 0). We have few small RoR apps that are not db intensive (and number of those apps is growing). We think it would make sense to put them all on single RDS instance and scale plan when needed. Case is what about Multi AZ? Will it add costs with every added db? Could we somehow cluster all those standby replicas on the same instance to reduce the costs? Ideal scenario in my mind would be one instance for all main databases and another instance in different AZ for all the replicas. Is that doable?

Answer 1

Case is what about Multi AZ? Will it add costs with every added db?

It basically doubles the costs for a RDS instance, but not for databases, as you seem to want host multiple databases on a single RDS instance.

Could we somehow cluster all those standby replicas on the same instance to reduce the costs?

Standby replicas are for the RDS instance, not individual databases hosted on that instance.