A post titled Supporting Custom Game Developers on the Dota 2 blog has word that Valve is taking a new approach to allow mod developers to monetize their work with a subscription system called the Custom Game Pass. This sort of thing has met with resistance in the past, and this time they are looking to avoid this by offering the mods for free, and using the subscription to enhance the experience. There's an interview with Gabe Newell on the subject on PC Gamer, and a FAQ with more. Here's word:

Introducing the Custom Game Pass, a new system for supporting and rewarding custom game developers who are committed to building vibrant and lasting communities around their games.

Custom Games Passes will be limited to a small number of high-quality games that have established themselves in the community. Purchasing a pass directly supports the creators of the custom game as well as granting you extra features for that game. All custom games will remain free to download and play.

The first Custom Game Pass will be available for Roshpit Champions at a price of $1 for a 30-day duration. While everyone can play Roshpit Champions for free, the Custom Game Pass will enable additional stash and character slots, and offer multiple perks for the duration of the pass.