While I'm sure Bethesda is thrilled about getting money in the bank from people selling mods, I find it wrong to charge for something that modifies an already existing product created by an entirely different party, even if the original creator is theoretically okay with it.
I feel like this will cause the modding community to splinter and become more hostile, it will get its share of opportunists and greedy assholes, but the mods that aren't free will see a much smaller distribution to users. At the same time people offering mods for free could see much higher popularity even if the quality isn't as high as some of the paid ones.
Of course I could also be wrong and people will happily pay for mods in the end. I just know that as an occasional modder I would feel wrong about charging for a mod for a multitude of reasons, and as a consumer I would find it extremely hard to justify to myself to buy any mod.