You can (and we do) sell commercial apps based on the Qt libs.
All you need to do is.
1, Dynamically link your exe against the Qt dll’s. so your product’s installer includes your app.exe and only the used Qtcore4.dll etc. This is a good idea anyway because you can upgrade your app without having to change MBs of dlls.
2, If you fix or change anything in Qt you need to offer the changes (to the Qt sources only) to your customers and/or back to the Qt people. This is pretty unlikely since Qt is very high quality.
The only reason to buy a Qt license is if you need to run on a platform where dynamic linking isn’t possible – perhaps a settop box – or you need extra technical support.
ps. Even if you don’t modify Qt you do need to provide the original Qt source if a customer asks for it. But this is just a matter of pointing them at the Qt site.
There is never any requirement to reveal your source code.