Even a database with no licensing fees costs money in terms of wages/salaried employee time to use, so while that cost advantage is real, there are costs on both sides. If MS has products you want to use that are much easier (read: cheaper) to use with their paid database than some free alternative, that’s certainly a good reason to consider it.
The longer you use it, the less likely it is to pay off, but execs focused on short term profits don’t weigh that very highly.