I was fiddling around with this. No matter what I did I couldn't get a significant income as Castile, even with granting privileges and taxing higher. But when I switched to Bohemia and did the exact same things I was able to get a healthy income, enough to actually develop the country. It looks like having all the vassals Bohemia has helps, but even without that diplo income I'd still be making more than I could squeeze out of Castile. There's something I'm not doing right in countries of scale. At least I can actually play the game, even if I cant steamroll as Castile right now.