KDE applications actually use the KDE libraries, which are built on top of KDE. If you want to use these, then you will have to release your code under the GPL.
You can, however, write a pure Qt application that is commercial (rather than GPL) if you wish. You will have to buy a deveopers licence from TrollTech (www.troll.no
As an aside, the GPL doesn't stop you from charging money for software. Instead, it forces you to (among other things) release the source-code and allow anyone else who gets your software to modify your code and/or redistribute it as they please. It's designed to allow you to develop the best possible software by (primarily) allowing people who find bugs or limitations to fix them, and share their fixes.