db.m9g.large is an AWS RDS size with 2 vCPUs, 8 GiB memory (AWS Graviton5). In US East (N. Virginia) it costs $0.183 per hour on demand ($133.59 per month) for PostgreSQL, Single-AZ; a 1-year Database Savings Plan costs $0.1464 per hour, 20% less. It is sold in 4 regions.
Prices by region
USD per hour, PostgreSQL, Single-AZ. Reserved prices are no upfront, standard. Cheapest on-demand first.