The best model is a PWYW (Pay-what-you-want) for mods with either a minimum of $1 or a true PWYW (so free, but if you want to throw some $5, go ahead).
It works fantastically well with even small obscure indie games on sites like
http://itch.io/
Modders put a lot of work into what they do, I'm not a modder per-se but I am a programmer and done my share of work on games in the programming department, I've written in-house tools for design that are later usable as modding tools. I know the type of work that goes into making mods.
The thing is putting mods strictly behind a pay wall is an issue of PC gaming culture/community when it boils down to it. For 20+ years now mods have been a thing on PC, it has created new games and genres and birthed game developers themselves, they were always community "first" aspects of video games on PC.
However, modders couldn't receive money legally but now they can which is GREAT but putting it behind a wall is tainting that culture of PC gaming. That's why I think a PYWYW model would work perfectly (and only a PWYW model and/or donations/patreon) because this way people can reward modders for their time and effort but also maintain the essence of modding as well. Personally I think 25% is way too low a cut. They do all the work and receive the least? Lol. It shouldn't be that way, it's absurd.
So in essence I support paying for mods via a PWYW model only. I don't support putting them exclusively behind a pay wall and neither do I support only getting a 25% cut for all the work. They deserve more.