At $14K/core [0] for the Enterprise edition, it doesn't fit exactly on my definition of cheap. Sure, it's 1/4th of Oracle's price, but still translates to 6-7 figures for even small deployments, which again is not cheap to me.
But I guess the main attractiveness for current SQL Server uses might be more on the licensing terms/compliance problems than the cost. Just trying to understand how to license SQL Server and how cores are counted already requires you to read a 42-page guide [1]. The risks associated from incorrectly purchasing licenses for your SQL Server cluster(s) may be a compelling reason to jump into open source with minimal migration costs (at least compared to migrating directly to Postgres or any other open source database).
[0]: https://www.microsoft.com/en-us/sql-server/sql-server-2019-p... [1]: https://download.microsoft.com/download/e/2/9/e29a9331-965d-...