Hm, I'm usually a fan of a lot of things Valve tries but I'm not really behind this one.
Don't get me wrong, I love modding and all the work that modders do to improve and expand a game's life and value. I'm also not against them receiving something for their hard work and enthusiasm, either. But I don't know if this is the system I'd get behind.
There's simply too many problems for me to really support it. I think I have to echo other people that suggest a "Pay what you Want" method only instead of having the creators list their own value. For me, I can not see buying a mod. Unlike a game, I'm just not assured that there will be support for a mod nor that it will work with all the other things I add to a game. With Skyrim, I've spent weeks getting it setup, tweaking/adding/removing mods until it's stable enough to function.
And even still, the game runs worse than if I left it vanilla. Despite my efforts to try and completely get rid of the infinite load bug, it still happens--and regularly too. I've found workarounds which let me still play but had a game natively carried that bug, I would have asked for a refund and told people to avoid it for being broken. I also have occasional crashes to desktop and it's definitely mod related.
I live with these simply because I understand that injecting so much extra into the game is going to produce unstable results. But it's all free and, if I want something that is stable, I can always run a clean playthrough without having lost anything.
I don't think I can be assured of this support with mods. And even if I'm just paying, say, a dollar for one--that's still a dollar wasted if it's incompatible and I can't play my game with it.
Had this been reserved for the Nehrim's of the mod scene then maybe I would consider it--but only after the game was "finished" by the developer and I knew they wouldn't release an update that would break any purchased mods I may wish to run.
Which is unfortunate because the idea is noble even if I simply cannot support the execution.
So many updates over at nexus going on.
So many "you may not use my stuff, if you are going to charge/monetize of it" combos being added to the descriptions.
There's this as well. There's been a few mods that I followed and used that came to a crashing halt over personal issues and modding drama. Throwing money into the mix seems like it'll only exacerbate those issues. Which is a shame because some of the best mods are large scale community created pieces.