You can develop commercial apps with PyQT, your code just needs to be compatible with the GPL3 license (https://www.riverbankcomputing.com/commercial/pyqt). PyQT is not different from any other dual-licensed open-source project in that sense.
It depends on how much time you will spend developing your app. I've been working on mine [1] for 2331 hours (I track my time). $500 is nothing in comparison.