Yeah, I'm ok with paid mods being a thing if the devs are ok with it, and there's some sort of system in place to easily allow DMCA requests when people steal from each other. Valve will need a well thought out policy under which stolen content can be kept out of the store though.
The thing I would actually like to see is some semi professional studios making mods for games. Sort of like unofficial expansion packs for around $5. However, I don't think that the average person should be getting anything for their mods. The way the community works at this point in time means that the system will fail.