How much do you pay a month for what size?
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).
This cost is kind of hidden since to estimate this in the early stages of a project is an art. In one project on my team the IO cost is about 8x more than cost of instances. But imo it is still worth and I never actually calculated how much we would pay if we were running on RDS + provisioned IOPS.
Right now, on a multiple of the traffic we had before we moved to Aurora we are paying less than half what we used to for IO.