Hmm, I need to disagree. MySQL is Open Source, you don't have to pay for it, not even for commercial use, as long as you don't distribute your application.
Read the licensing on the site; it IS dual-licenced. You can even redistribute your application, but it must be licensed under the GPL.
I have not read the entire commercial license, but it comes with support and (I believe) a warranty. The commercial license is only required when you want to redistribute MySQL as part of your commercial software package, which makes perfect sense.