Comment by illumin8

This is because you are using SQL Server. Microsoft has intentionally made cloud pricing for SQL server prohibitively expensive for non-Azure cloud workloads by requiring per-core licensing that is extremely punitive for the way EC2 and RDS is architected. This has the effect of making RDS vastly more expensive than running the same workload on bare metal or Azure.

Frankly, this is anti-competitive, and the FTC should look into it, however, Microsoft has been anti-competitive and customer hostile for decades, so if you're still using their products, you must have accepted the abuse already.