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.
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).