I think other commentators are quite right to highlight all of the extra non-core RDBMS abilities that are bundled with the commercial solutions.
Here is a matrix of Oracle editions and features that would be worth browsing just for an understanding of the “extra” features, particularly in the context of what it would cost to develop and support your own version of them (if that is even possible) on a product like MySQL: https://docs.oracle.com/cd/B28359_01/license.111/b28287/editions.htm#DBLIC116
For example, if you had a requirement that said: “Users shall be able to recover any previous versions of data that they have deleted/updated up to one week after the changes has been committed” then that implies a certain development overhead that I think would be much higher on a system that did not have a built-in feature like oracle’s Flashback Query.