If a modder has done a really great job and has created, for example, a well designed 30 hour campaign with good quality voice acting, I don't see why they can't have the choice of selling their "mod"... which would sound in this case more like an unofficial expansion.
But ideally the share of profits with the developers/publishers should be based on what the mod maker earns after they've recovered all their costs incurred (including tax on earnings), which would complicate the whole system and require humans to get involved to audit everything.
So it's probably better to just go with something like a 80/20 split (80% for the mod maker, 20% for the rest). After all, if the modder is good, he's effectively increasing the demand for the base game and thus increasing their sales, as well as bringing more traffic (AKA potential customers) to the site that hosts the mod.