Pick the data model first: managed PostgreSQL for most apps, MongoDB Atlas for documents, CockroachDB for multi-region writes, and Aurora if you already live in AWS.
Entry prices from vendor pages run from $5 for a PlanetScale Postgres single node to about $203 for CockroachDB Standard, and security features such as private networking are often gated to higher
For 1 TB and 2 always-on ACUs, Aurora I/O-Optimized only beats Standard above roughly 903 million I/O requests per month, so measure your I/O before switching.