The main problem with gtk is that there's no central point of all the information you need to completely understand it. GObject, GType and signals take a long time to learn, and i only did that after reading the gtk lists for months. This probably doesn't matter as much with gtkmm which has C++ objects. The gtk source also has an examples directory for testing lots of functionality, but it doesn't help a lot for someone trying to learn the basics.
That's exactly what my point was: the time it takes to get up to speed, and the time you need to invest to keep up, easily costs a multiple of the 3000$ this whole discussion is about.
cheers, roel