@pasik
Well, that's a relief.

No mod purchase system, phew.
@SquellDay
It depends on how it's set up. It's generally split into some groups: the money the modder gets, and the money the modder doesn't get (which is further split into the money that Valve gets and the money that the developer gets). For example, with Skyrim (the only game currently to have the system, which seems to be backlashing horribly), the modder get a flat 25% cut (which to be honest is a rather pitiful amount - compare that to the Apple app store, which gives 70% to creators), Valve gets 30%, and the developer, Bethesda, gets 45%.
Oh, and furthermore, if the developer has a producer behind it (for example, how SEGA is behind the Total War series), the producer gets the final word. The developers can't even do anything at that point, let alone the modders themselves.