What is the cost of using NULL in database columns?

Question

I use MySQL and SQLite often and plan to add PostgreSQL to my workflow soon. With that in mind, what are the costs of using NULL in each database? I have heard that MySQL adds an extra bit to each value of the NULL column to mark it as nullable.

Answer 1

This question was answered separately for PostgreSQL: How much disk space is needed to store NULL values ​​using postgresql DB?

and for MySQL: NULL in MySQL (performance and storage)

But for repetition, they both use bit masks to mark zeros.