Open source is not "free," but it is much cheaper: the numbers
Trading the annual check to Oracle for a team that knows PostgreSQL usually comes out well ahead.
PostgreSQL and MySQL in their community editions charge no license. Zero. But "no license" does not mean "no cost": you need support, people who know how to run them, and sometimes a migration. Even counting all of that, the numbers almost always favor open source by a wide margin.
01 The license is zero, and that alone changes the game
PostgreSQL and MySQL community have no license cost and no mandatory support cost. What you paid Oracle or Microsoft to license disappears from the equation.
USD $0 in license
02 The cost shifts to operations
You pay for optional professional support, for talent to administer it and for the initial migration. It is a real expense, but a controllable one and far below the license.
03 Migration is a one-time cost
Moving a database from Oracle to PostgreSQL takes effort up front, but it is an investment that pays for itself in a few years with what you stop spending on licenses.
04 Not everything migrates equally easily
Databases with a lot of proprietary Oracle logic (heavy PL/SQL, specific functions) take more work. It pays to measure before promising a saving.
// An illustrative case
Imagine a company paying USD $167,000 a year in Oracle support. Migrating to PostgreSQL costs, say, USD $120,000 one time and USD $40,000 a year in professional support. In the first year it nearly breaks even; from the second year on, it saves more than USD $125,000 every year.
Illustrative example with typical market figures, not a specific client.
// next step
The savings are real, but they depend on how tangled your current database is. A feasibility scan tells you what the migration costs and how much you save. At dba.mx we quote it at a firm price from the start.