Valve is updating Steam today to allow users to charge real money for game mods on the Steam Workshop, starting with Skyrim and rolling out to other games (contingent on the approval of their developers/publishers) in the weeks ahead.
This opens up more avenues for budding and established game developers to get paid for their work -- as former SimCity artist Bryan Shannon is by creating Cities: Skylines mods -- and furthers Valve's long-stated aims of democratizing content creation for games on its platform.
Update: A Valve representative responded to Gamasutra's request for clarification on revenue splits by highlighting the Skyrim Steam Workshop page, which notes that all revenue from sales of Skyrim Steam Workshop content will be split between Valve, Bethesda and the content creator(s).
People who sell Skyrim content on the Workshop get a 25 percent cut of the revenue, but it seems that the amount a content creator receive is ultimately up to a game's publisher or developer. New supplemental terms for the Steam Workshop legal agreement state that "the percentage of Adjusted Gross Revenue that [content creators] are entitled to receive will be determined by the developer/publisher of the Application" for which they've created content.
Presumably, content creators work out the details of that revenue split when they give their payment information to Valve, at which point they can choose to set prices for their mods or allow their patrons to pay what they want. They can also choose to send a set portion of their revenue to a Valve-approved user or community, like the Nexus Mods hosting platform. More information is available via the links above, as well as Valve's dedicated page for Steam Workshop paid content.
Valve is also offering a 24-hour refund policy on all Steam Workshop mod purchases as it continues to incentivize user-generated content commerce, having earlier this year allowed Steam Workshop users to charge money for in-game items (but not mods at large) they created for non-Valve games and then debuted its new Source 2 engine with a stated focus on user-generated content.