When I looked into it back in 2004, it was just over $6000 USD per developer for commercial use. Crazy money. At the time there was already an "open" license I believe, so what's changed since then in terms of licensing?
Is it currently possible to develop a commercial product without paying over $6000 per developer working on your project?
It was licensed either under the GPL or the commercial license. Since 4.5 (after Nokia purchase) they've also added LGPL to the mix.
To answer your question, yes, since LGPL is now an option you can develop closed source proprietary commercial software as long as you only dynamically link to the Qt binaries and that any changes you make the offer of the source to any changes you may make to your build of Qt.
I also thought the commercial licenses were a tad on the expensive side, but since I've been using it I realise why it was expensive. It's good, it's very good.