Go check out the Nexus site for Dragon Age Origins. The list of bug fix mods is long. The list of fixed bugs exponentially longer.
BTW thanks for butchering the context of the DayZ reference. It was referencing mods, not platform specific sales so altering it's context to suit your argument was rather poor form. 1.7 million in sales does highlight that the PC platform isn't as dead as a lot of people think. Not to mention a lot of the time when multi-platform games are concerned the ports are down arse-backwards, i.e. the games are developed with high detail assets on the PC, then reduced in detail and quality until they are at a viable point for the console platform (usually lead being X360, now PS4/XBOne) with the PC then being an upscale port of the Console game. PC specific UI's and Options are usually done late in the day and often poorly executed due to time/budget constraints and a massive lack of care due to it not being a massive market. Thing is how many of those sales would have been transferred or even doubled over to PC if the quality of the PC game matched that of the console?
Very few. Cross platform purchases happen, but in the thousands, not in the hundreds of thousands, let alone millions. And I never said PC gaming was dead. Only that it is a smaller group than consoles, not only as a whole (where console dominates PC in terms of numbers) but also juxtaposed against individual platforms.
There are more PC gamers out there in total than console gamers. They are often people who have consoles too, but if a game is great on a console and half-assed on the PC, they'll buy the console version. Personally I'm a PC gamer as I can't stand the interface of consoles normally, plus I can't mod or fix problems I encounter, which on a PC I can do.
Unless you are trying to make the case that everyone who owns a PC is a PC gamer, I have no idea where you are getting this impression from.
For games that are cross platform (console and PC), consoles outsell PC. For standalone PC games, there are next to no titles that can compete with the biggest sellers of the highest selling console games.
If sales are lower on PC than on consoles in nearly every method of slicing it, then how can you say there are more PC gamers than console?
As for paid mods, this is a good and bad idea. Some mods are indeed worth a buck or two (Dragon Age Redesigned for instance has hundreds of hours of work put into it), yet others wouldn't deserve it. It reminds me of when a bunch of Russian modders made a large mod for an old game and tried to charge for it. They were rude and arrogant about it, then eventually sold the idea to the publisher of the game and released the mod as a standalone version of the game. The sales of it tanked as the reputation of the modders involved were known by most of the community already, not to mention they refused to release patch fixes after claiming they weren't getting paid. That's the dangerous part of getting money involved with modding, for the most part modders do it because they want to improve the game they like, the ones doing it for fame and recognition generally don't get it 
I agree that it would be a thorny path. However, with a game being distributed through Steam or Origin, you could have a built-in way to monitor which mods are installed and if the gamer had paid for it or not, along with a payment system that would keep financial transactions easy and convenient for the gamer, modder and developer.
But I firmly believe you'd let the market determine how such things would play out. People who overcharge for crap, who don't update their content, who are completely unknown and don't have a reputation for quality... all of these things would be reflected in their lack of sales. Modders with excellent content, who work quickly to identify and fix bugs and who have a following after releasing quality mod content would be more successful. Just like with anything in life.
And, of course, the modder would set the price. If they want to do it as a hobby and not charge, they can. If they are brand new and want to test out some mods or try and earn their name, they can do so wih free content and look at paid content later down the road if they so choose. And with the developer getting a small portion of the proceeds, this would not only encourage more games to include modkits, but also work to give the most robust support possible and encourage more to enter the market (contests, tutorials, file structures, etc.). And, of course, it makes bundling mods and porting them to a console a possible reality, seeing as how the developer will be getting some revenue from doing so that will more than offset their console certification costs.