erm not neccesarily so...
if you use MySQL then depending on the licence you use you may have to pay for it.

its free if you release the source code of your application under the GPL
If you bundle it as part of your application then you have (well ought) to get a commercial licence for the server. the premise from MySQL is if you are making money by using MySQL then they want some of the money aswell, whihc seems fair enough in my books givent he amount of time energy and money that has gone into developing MySQL

I'm not to sure where the dividing line falls if you ship an application which can talk to a MySQL server and the organisation/person installing the App installs their own copy of MySQL.

equally I'm not to sure where the dividing line is if you develop software inhouse targetting an internal MySQL db.

however if it was a critical db application then you would have a commercial licence, wouldn't you