The Unreal Tournament community has already been dealing with similar issues, since Epic's Marketplace downloads can be monetized and may include addons for the new UT. There may have been some interesting discussions from there (idk since I haven't been active there for a while). Although I think Epic gets a much smaller cut from things sold on the Marketplace- like 30%.
TB raises some good points, as always: well-implemented monetization could encourage skilled modders to create more high-quality work, and modders being paid for work is in principle a good thing.
OTOH, given the existing problems with Skyrim's modding scene in particular, this could lead to some egregious money-grubbing on the part of greedy modders (Steam Workshop is already filled with stuff stolen from Nexus, hopefully monetized mods will have more oversight here). Not to mention that, as TB brought up, users have no recourse whatsoever for refunds after 24 hours. It's acceptable an even expected that some mods be buggy and even broken at times - this is much less acceptable when the mod is a paid product.
Also, something TB didn't mention but I've seen talked about, not entirely sure if it's true: but monetized mods on Steam would have to make $400 gross before the mod author sees any money ($100 at that point) at all. Considering that many authors will attempt cheap cash-grabs like a single sword model for 99¢, which may not be downloaded by many people, they may not get any money from it at all - it would go entirely to Valve/Bethsoft. On the whole the system seems very exploitative both of users and modders.