I'm not sure what to think about that.
On one hand we have plugins/behaviors like: Litetween or magicam, which are made of stuff you could do in events. But were made to make our life easier, so instead of writing xxx number of events you simply add it to you project.
On the other hand they are things like Spriter (or should I say Spriter reader/encoder?) or Q3D which are adding things you can not do in C2 natively.
And there are effects, which are completely different to those stated above.
I think having a category for plugins and behaviours would have much more sense with C2 modularity feature.
For now i would say that 5$ is a good price for simple and intermediate plugins/behaviors. And more complex ones should be approved first and goes with special permission.
But in the end it all depends on developers. One can price Fade behavior to be worth 0.99$ and the other 10$ :/