The profit margin on any of their plugins is phenomenal. He said as much in the interview when they first put out the Darkglass plugin.
What's even more phenomenal is how often they're able to reuse components and get people to pay for tweaked versions of the same amps and effects again and again. The ROI on their model development is exponential at this point.
I don't think that people are necessarily happy buying paid DLC for their modeler. If anything, I think their plugin audience dwarfs the Quad Cortex one, which is why they take care of the plugin people first and always. The minivoicer, the synths...that stuff drifts to QC eventually. But not before they've squeezed the juice out of it with plugin sales.
