Some mods like Morrowind or Oblivion for Skyrim which are years in the making and massive I would happily pay for depending on the finished product, But not every mod deserves money.
Allowing modders to have a donation button rather than a purchase button within the Steam workshop that a majority of the proceeds go to the modder and a small % to Valve who host the files would be a much better idea as I can see these paid mods putting a lot of people off and slowly veering away from the epic scene that is modding.
This way Valve still get a nice chunk of change for simply hosting the files.