TL;DR: A realistic Oracle to PostgreSQL migration costs between €5,000 and €80,000+ depending on schema complexity, PL/SQL volume, and whether zero-downtime cutover is required.
The tools are free.
The cost is almost entirely consultant time, testing effort, and application changes.
Most medium-complexity migrations pay back their full cost within the first year of Oracle license savings.
Oracle to PostgreSQL Migration Cost: A Realistic Breakdown
