The way modding is right now is perfect, at least from a user's perspective. You get everything for free but if you can afford it and want to then you can donate to the creator. There's no need for a paid system because that excludes people who can't afford to pay for the stuff.
Mods would likely increase in quality if there was a financial incentive. Many gaming communities have both free and paid mods available and the paid mods tend to be much better. Assetto Corsa immediately comes to mind.