I imagine we'd see a lot more quality mods a lot quicker if the source and result were free. I'm pretty skeptical of charging for fan packs. I suppose that's not really an option if you've got any proprietary stuff in your code though.
I'd also be a little concerned about who gets paid what when it comes to the mod group. There's a lot of potential for DRAMA. If twelve fans work on a mod, but two do the vast majority of the work do they all split the mod profits equally? Who decides? I doubt Paradox can or would want to police this. As soon as you introduce money you introduce the potential for a lot of anger, in-fighting, and weirdness. Particularly, introducing money to a internationally distributed, ever changing group of fans who aren't, in any sense, a legal company or organization.
Also, if it's a free fan pack, people don't have much room to complain about bugs and problems. But, if you're selling a product on your webpage there are certain (reasonable) expectations of technical support and bug fixing, which Paradox couldn't supply and the fan group might not be able to keep up with (assuming they don't dissolve after release).
If it were up to me, I'd say, yeah, only give the code to organized groups who sign the NDAs, but don't pay them and don't charge for them on Gamer's Gate. Anything else strikes me as a bad idea with a lot of potential for angry fans and frustrated consumers.
Not to rain on the parade, but it seems to me fan mods should remain a labor of love, not a for-profit endeavor, for a lot of reasons.