If this creates a mechanism for mod authors to be paid, then that's neat I guess. If even one penny is going to Valve/the game devs, this is reprehensible.
It's a big "buyer beware" type situation to be sure. I think the thing I'm responding to most is that they offer a "pay what you want" option. Actually needing to pay for a mod is shitty, but I expect in that case what'll happen is that most people won't buy it, and therefore the mod won't be very popular. And basically the only way to browse the workshop is based on popularity, so nobody will ever see the mod. There's a built-in incentive here for mod authors not to charge much/anything for their mods.
IMO the game maker deserves a cut, after all they did make the base product the mod is for. Without the work of the game studio the mod wouldn’t exist.
At least 70% should go to the mod maker since they put in the effort to make the mod. Valve and the dev studio can split the 30%, valve deserves a cut as well since they’re maintaining the storage and delivery of these mods and presumably facilitating the financial transactions associated with these paid mods through their store. Storage and network usage isn’t free and neither are running CC transactions which people seem to forget.
The game devs have already been paid from you buying the actual game. They shouldn't get to profit off of other people's work in the form of mods that they put no effort into creating.