They modder will get 25% of the money you are paying. What do you say for that?

Still better than the 0% they'd get before. Although it depends if the mod creator is the one who gets to determine price and whether or not they can monetize their mods through platforms other than Steam (host their own website where you can download the mod.) 

I can see this getting abused. I like looking through mod to see how I can boost the experience but I can see people charging a euro or to for small things. It will almost ruin the whole thing imo

Wait for third party publishers to find ways to ask for their share too. It's their game afterall.

For selfish reasons I'm a little bummed that many mods may now no longer be free, but to be fair a lot of them are so packed with content that paying would be reasonable.

I would really like to know the legality and payment structure behind this. Even though modders do create new features for these products, they still don't own them. I'm very curious to see how publishers will react, and whether or not they see revenue from this.

I imagine a part goes to Valve, the usual 30%, the rest to the publisher/developer. While the modders should get about 30% I don't particulary see a problem with it.

Here is the payment info FAQ, it's rather long.

Is it not implied that the publishers/developers already get a piece of the pie? The 75% is split between them and Valve I assumed. If a person doesn't want to monetize their mod they don't have to, to be honest. No matter how you try to spin it, this is another option in which a person who wants to monetize their mod can. And if the publisher/developer allows them to monetize it, I'm sure they don't have to go through Steam to do so. They can make their own website with all of the costs involved in doing so, pay costs to advertise their mod, and then sell it from there. Of course while still giving the share that is due to the publisher/developer, per request. 

I mean if a mod like Skywind sold for $20/purchase, then the developer would get $5 /purchase if they chose to use Steam as their publishing/advertisement platform. Now let's say that it sells one million. That would be $5 million dollars given to the mod developer. And then let's say they chose to reward the participants, who are currently doing it for free on their own free time, based on how many hours they put into creating the game. Let's say there is a total of 100,000 man-hours (100 people doing 100 hours each) spent on development. That would be $50 /hr - about as much as a video-game developer makes. Or if somebody spent 100 hours of work developing the mod they will be compensated $5,000 before taxes. That's pretty nice for something they were doing for free before. 

Why should the main game developers/publishers get money? Because they created the engine, tools, art, IP, storyline, quests, etc that the modders are creating from. Without these things there would be no mods. 

Oh my god.... I thought they're joking...

Not a good sign in my opinion. Hopefully other PC games don't follow suit.