The .NET runtime is a part of Windows which costs money.

Mono falls in the same category as OpenJDK: Not relevant for this discussion.

The money Windows cost are nothing compared to Oracle fees.

You can buy Windows server editions for less that a developer's week salary. You'll need MUCH MUCH more than that for Oracle.

It's very unlikely that the license to run the commercial JVM will cost the same as a license to run the Oracle DB. These are two very different beasts.

In fact, I would guess that the cost of a Windows server license would be something of a benchmark for the price of the commercial JVM license.