I thought developer tools would be the toughest to make money off. Developers always believe there is a free, open source alternative out there somewhere, and if not, someone's sure to start one soon.
All developers don't always pursue free tools; I suspect you can make a very nice living selling tools to a small percentage of developers.
Personally, I've paid money for the WebStorm IDE, the PyCharm IDE, Sublime, Dash, Paw, Balsamiq, and more. I've bought ready-made UI components, database drivers, etc., and I'm about to start shelling out $4,000+/year for a commercial Qt license even though I could probably get by with Qt's free LGPL version.
And that's just me as an individual; companies are even more willing to spend money on developer tools.
It's interesting that you as an individual would want to buy a $4,000/year Qt License. Are you trying to found a startup that uses it or do you have some other reason?
Yes, I am developing a product to sell, and I hope it will become a profitable venture. I would not pay for Qt's commercial license if it was just for hobby projects. (For clarity, it's not necessary to buy a commercial license for Qt just because I want to sell Qt-based products, I could do that under the terms of the free LGPL license, but there are a few nice extras that come with the commercial license and I consider them worth the expense.)
