I've already laid this out on two other forums, but I think the issue isn't with the idea of some mods costing money, but with Valve opening the floodgates without any overhead. The two big issues with this were that Bethesda's cut was ludicrous (Valve getting 30% is fine since that's what they make on games) and that it was bound to be a copyright clusterfuck. The latter can't be stressed enough to this community in particular. Remember when Acumen made a tractor model based off someone else's concept art, and Valve made Rexy design and model a new one at the last minute for Mann Manor because the concept artist couldn't be contacted? That's the level of diligence that should have been brought to the table here. Not just letting anyone post something and slap a price tag on it and say "Well if you think they're stealing from you then file a DMCA notice!"
And speaking of the DMCA, let's fuckin' talk about that, shall we? Under the terms of the DMCA, it is illegal to sue somebody for posting/allowing copyrighted content on their website without first sending a takedown notice, and waiting for them to refuse to comply. First off, how the fuck are you supposed to send a takedown notice to some assclown on Steam who doesn't even provide a public email address? Secondly, I can only assume that this doesn't apply if they're charging money for your stuff, because what if somebody makes a buttload of money off it before you have a chance to stop them? There has to be some recourse other than telling them to stop and letting them keep the money they've made up to that point. The MPAA and RIAA lawyers who drafted the law never would have signed off on it with such a big loophole.
I agree that, on principle, there are some mods substantial enough to be worth money if their creators so desire. But for those, Valve should have done what they do with the Source mods they already distribute on Steam: cherry-pick them (or have Bethesda cherry-pick them) and put them on the Steam store, and leave the Workshop for free stuff only. There are a couple mods that I would even agree that Bethesda deserves a cut of, the ones that recreate Morrowind and Oblivion in the Skyrim engine. Assuming those mods are even out yet; I'm not exactly up to date on this stuff.