Honestly, if you mod for money you're not a real modder anyway. You do it out of passion, because you love a certain game and want to see more of its kind. Or because you want to have a certain thing within the game that wasn't originally in it, so you put it in.
I remember those unofficial "expansion packs" back in the late 90s which were basically just fan-made levelpacks sold in stores. They were usually worse than what you could find for free on the internet. I bought one for Age of Empires and one for Star Craft back in the day; the quality of the campaigns was, in each case, much worse than the quality of the original campaigns, and worse than the quality of good user-made ones, too.
Successful modders can easily get into game development as a job and earn money that way, sure. They might even, after finishing their modding work, go on to create a full-fledged indie game of their own. But they don't start modding for monetary reasons, the money flowing in is just a result of their dedication to the thing that started out as their hobby. It is a reward for their passion.
Look at the top quality total conversions of the last two Elder Scrolls games by the German SureAI team - Nehrim and Enderal. They feel like they're entire games of their own, and yet they were made completely for free. They feature new models and textures, a large and completely hand-made world, and even FULL FUCKING VOICE ACTING. And they were made for free, as a hobby project. I very much doubt that paid mods will attain that same level of quality, because the people behind paid mods are motivated more by greed than by passion.
Just look at the average RPG-Maker game for sale on Steam for between 90 cents and 3 euros. Most of them are utter shit, an attempt to make some quick cash by releasing a shoddily-made game and hoping to get some sales when they price it at 90% off during the Christmas sale. Now, I don't like JRPGs overly much and haven't played many RPG-Maker games, but I'm willing to bet that the most highly rated of free games made with that engine are over 9000 times better than the shit some people dare to sell for money.
Paid mods are a mistake. A huge mistake. Nothing good is going to come from this.