Going back the the paid , or "premium" membership, if its priced less then a kit, I think it might work, but you would have to give those members something special, there is one forum that I am on an they offer premium membership, its $12 for one year I think, and $25 for a life time, with that you get a members only section on the forum, you get to control more aspect of your profile and post/threads, and some other things
If the price is right, and you offer some goodys, its something to think about , would I be willing to pay say $12 a year , sure , but not $25 a year .