Really good open source DB software like MySQL (MariaDB) or Postgres can handle that load with a tiny fraction of the operational costs.
Kind of... MySQL is really not comparable to Postgres, at least as MySQL is generally used. It's commonly set up to sacrifice ACID properties in favor of performance. When configured to go as full ACID as it can, its performance and robustness are inferior to Postgres. MySQL/Maria is great in its little niche, though. And I suppose the same could be said about Mongo.
Oracle will collapse and fall over dead.
From your lips to God's ears.