I'm not against paid mods. If someone makes a good mod then there is no reason he shouldn't get a little something in return for the work he's put in. And besides its not like everyone will start asking money for mods all of a sudden.
The problem with this is, that you open a can of worms and my gut feeling tells me, it won't be good for modders and mod users in the end and leave you with a system and community that will be worse for everyone.
See, I am not against people geting money for their hard work. But, you have to remember what sounds awesome on paper, doesn't have to be awesome in real.
One problem, that first, you're always creating content on the shoulders of others. If you're modding for Skyrim or Fallout 4, you're modding on those games. You're using their assets, engine and creation kit - most probably. Admittely this isn't so much of an issue, as it is more just something that should give us food for thought. But I think that it is a big difference if you're making something completely on your own, or if you already use the content of others.
The other issue is, Bethesda (and other companies!) might get money, for people fixing their shit while doing nothing. And I don't even want to start on going on about how shit that sounds. It simply should never become even a possibility. Bethesda games have already the problem that they are barely tolerable without mods. Don't give them even more incetives.
Another point. Mods are not professional content and/or products. I am sorry to say this, but as much as I love mods, the one that actually can compete with real professional content, are very small. This is not so much of a problem if you're offering content for free. Hey?! What are you complaining about! Don't like it? Don't use it! It is free after all! Bugs, gameplay issues, etc. all of that can be somewhat ignored here. Within reason of course. But ... not so much once you demand money in my opinion. The way how modding works now, it is not a professional work environment, this has advantages, but it also has disadvantages.
Which brings us to another problem. What if you're not happy with the stuff you bought? What if someone decides to give up on a half finished project? Or if it's so full of bugs that you can't play it really or if it is destroying save games? How do you make sure that the quality controll is there. And what is the correct pricing anyway? How do you make sure that you don't have lots of people coming in, who want just to make a quick buck? And what if a mod is using content of others? Should they get something as well? Waaaaay to many open questions here. You could as well start to monetize every website on the internet.
Let us be real, if it would be actually beneficial and easy to do, we would have already mods for money. But I kinda feel it defeats the purpose of mods in the first place. They are not meant to be full games or professional content. At least not the way how I see it. I know a few games which started as mods, and became comercial products at some point. Like Counter Strike, Tactical Ops, Day of Defeat, Red Orchestra. But those are usually the exception rather than the rule.
What I think is a lot better and really fair, are donations without any obligations. That way people can go, test the content and decide if the quality is good enough to support the modder.