As a PC gamer though, I am used to trying out mods for free because they added so much extra value to the base game. When you have to spend money on this extra piece of content or that additional feature, then it starts to feel like DLC. Also, usually there's a limit to the scope of mods and it's not very common to see a mod of a game become an entire game itself. If enough resources were spent making a mod that warrants a price tag close to an entire game, then perhaps it would be more worthwhile to spend that money on a new game, IMO. Paid mods may also conflict with another paid mod. I would be very frugal or hesitant to pay for multiple things and only find myself using 1 of them. Unless, for example in League of Legends where you can purchase skins, you can alternate between the skins you own, with the main advantage that other players will also be able to see the skin you are using. For a Skyrim mod though, since it is a single-player game, only the player using the mod will ever see it, which will affect the perceived value of the mod.
Also, what is to say that people won't pirate mods or re-upload them elsewhere, where they can download them for free? It's going to be impossible to regulate. Right now that doesn't exist because a lot of mods out there are free to download. Additionally, since according to Valve right now, creators only get 25% of the sales, if they price a mod too high, not many people will buy, whereas if they price too low, they will make virtually nothing. I think either way nobody will benefit, the creator nor the user. And putting a price on anything makes it much more frustrating to the player who just wants to download the mod and get on with it. What if they buy a mod, try it, and then realize they don't like it? Would they be able to return their "purchase" and get their money back? I suppose there are no refunds on game purchases through Steam either, correct?
I think if a mod marketplace was implemented correctly, we would inevitably have some people who create the first (and/or best) mods that fulfill a particular role/niche, stand to profit the most, while others late to the scene or who create mods that are not as high in quality, make next to nothing. I could imagine there being a "mod monopoly" where only a handful of professional modders create the mods most people would use or want to purchase, until another modder creates something else at a cheaper price. I dunno, I'm just speculating.
I know that Valve lets creators make custom skins or item sets for DotA 2 where creators get most of the sales revenue, however last I checked there was a long line up for content to be approved. What works about that at least is that nobody can really pirate those mods, and it works similar to League of Legends' micro-transaction market for skins.
All in all, I think this whole idea of paid mods is more trouble than its worth. I don't think the creator nor user will benefit much. In any case, the creator should at least take a majority share of sales revenue. You don't see Adobe taking a cut of the pie from every artist who uses Photoshop to create whatever artwork they create. Having said that, what might be a better option is that modders may need to pay an initial fee to gain access to creating and publishing mods, but after that they should be able to receive a majority revenue share, if not all. I guess we will just have to see how it plays out.