Actually, providing free and paid configs doesn't seem like a bad option (unless some people are stealing other people's work and reselling it). Let's say there's a paid config of ARK, and someone decides to create a free one that contains practically the same features. The guy who offers the paid version will need to make his config unique in order to get sales. Of course there's some cons to this, but it could potentially be a nice way of getting some truly advanced configs.
But, it's going to have more cons than pros - especially because taking someone else's work and claiming it as your own is too easy. And it's probably going to create a lot of arguments.
You could use the resource manager in XenForo to provide paid themes, however - that's not something that I would mind paying for.