What is the estimated cost of migrating 15 GBs Oracle to RDS Postgres using DMS

For this use case, you will need to consider the following:

1. DMS Service - Free, as part of free tier
2. Schema Conversion Tool - Since you need to migrate from Oracle to PostgreSQL. This is free as well. check it here https://docs.aws.amazon.com/dms/latest/sbs/chap-rdsoracle2postgresql.html
3. DMS Replication Instance - this one you will have to size based on the requirement https://docs.aws.amazon.com/dms/latest/userguide/CHAP_BestPractices.SizingReplicationInstance.html
4. Data transfer charge - All data transfer into AWS Database Migration Service is free, and data transferred between AWS Database Migration Service and databases in Amazon RDS and Amazon EC2 instances in the same Availability Zone also is free.

So in nutshell, only thing that you will be charged for will be for EC2 instances and ebs storage for replication instance and S3 storage which will be required to store data from schema conversion

you can calculate the costs estimations using the Simple calculator at https://calculator.aws/#/

there is some things to consider:

• type of DMS instance
• time will take to migrate
• storage needed on the DMS instance
• data transfer