First time I hear someone claim RDS is cheap. What specifications do you have in mind?
Scaleway costs approximately 51 % less, looking at on-demand postgresql instance with 8 GB RAM (db.t3.large vs. db-dev-l).
https://aws.amazon.com/rds/postgresql/pricing/?pg=pr&loc=3
https://www.scaleway.com/en/pricing/managed-databases/#cost-...