Cost is complicated because we autoscale from 2-3 readers up 10+ readers during peak traffic. Instead of running 10 replicas 24/7, we can spin one up and within about 10 minutes it's handling reads. So we have a few instances that only run for about 3 hours a day and others that run maybe 6 hours. That is a big cost savings over running them all 24 hours a day. We couldn't bring up new replicas like that when we were on normal Postgres RDS.

Pricing is also surprising compared to vanilla Postgres RDS because reader nodes double as spare writers. A multi-az deployment of Postgres RDS plus two single-az replicas is more expensive than a 3 node (1 writer and two readers) Aurora cluster. E.g. on 2xlarge instances, this Aurora setup is $3.48/hour vs $4/hr on RDS for similar effective hardware and fault tolerance.

Running directly on EC2 is going to be much cheaper obviously. $1.51/hr for three 2xlarge instances(if you want to failover to an active replica) or $2.01/hr for 4 if you want a dedicated failover instance (like RDS does).