If a mod comes out, and it is like let's say... the Forgotten City for Skyrim, a micro-expansion, i guess? I would happily pay money for that. Forgotten City is very good, content dense, and expands upon existing lore in a pretty good way. It is well enough written I'd consider paying $5.00USD for it.
I came to this conclusion on the basis of the following:
* Forgotten City has high quality voice acting.
* Forgotten City has good, rich content that can be played through slightly differently each time, with different endings. Thus, its replayability value is quite high.
* Unlike a full worldspace mod, it occurs almost entirely within a Dwemer Ruin or two, and like I said, this means the content is densely packed without feeling crowded.
* The writing of the quest is top notch.
Etc, etc, etc.
Something like Falskaar I wouldn't pay for though. This is because:
* Mixed quality voice acting and relatively sparse content. There is plenty of content, but it is spread across a wide worldspace.
* Of course, this is kinda moot because it was written as a portfolio project anyway.
A massive worldspace mod that expands upon the game as well as the official DLCs, or one that adds significant content to the existing worldspace would be totally valid to purchase at a level comparable to other official expansions. Indeed, maybe publishers could do something like Kingmaker for NWN: buy modules, polish them up to developer standard, sell them in a package expansion, give most of the proceeds to the mod authors, or bring them on board as new talent should they prove good enough.