I like to think that modders (at least in this community) do it for the love of the work and the love of the sim. If they did decide they wanted to be paid, they should realize that the level of not only the product, but the after-purchase support would have to match its status as payware. That in and of itself could make it more trouble than it's worth. And I would definitely not want ED to make one cent off of their work.
Me personally? I'm a pretty cheap bastage. The mod would have to really blow me away for me to feel like it brought enough value to an already great sim for me to shell out money for it.