For Aurora we are on a db.r4.8xlarge, which is ~$3340 per month for the instance alone. Aurora also charges for IOPS against storage, and we pay about $350 per month for those IOPS. Our DB is ~400 GB, so ~$40 per month for that as well.

I am pretty sure we could live comfortably on a db.r4.4xlarge, but we have some bursty analytics applications that sometimes bring our load up for about a minute, and we like the peace of mind of headroom during peak hours (we are in edtech with super predictable traffic throughout US school hours).

Total side note: I just went to the pricing page and it looks like they just released db.r5 instances for Aurora (https://aws.amazon.com/rds/aurora/pricing/)! It might be time to try those out. I don't see an announcement just yet though...

We have some production PostgreSQL instances running on t2 instances, they are only ~100gb but with the optimized queries we don't exceed CPU credits and the execution time rarely exceeds 200ms. Our requests per second on these are < 50 tho.