The reducing cross-AZ data transfer savings on one service resulted in a low 6 figure per year savings. Its something we overlooked during initial setup and now its something I check for when dealing with AWS networking.
If RDS or other “hard to replicate very quickly during disaster” infra is being run I personally would still have cross A-Z replication at minimum, to reduce network costs I would configure the “other zone” as a backup replica only and not for performance clustering.
With automation we can spin up full new compute stacks, including load balancers and DNS in about 5-10 minutes per “unique” environment configuration.
While it guarantees we could never have a no downtime failover, we’re okay with it and have more than halved our network costs (which admittedly were about number 8 on our AWS bill by cost).
If AWS has a regional outage this service is so far down the list of services to recover/restore that it probably will be overlooked. Accept the increased risk for the cost savings since it meets the reliability requirements of the service.
