I think there would be a significant increase in the site quality immediately, but I think it would be somewhat short-term. I don't think that fee can guarantee that the site will retain quality for the duration of that membership.

I would be curious to see if spammer accounts still get set up. I wonder if they think it's worth $5 to go on a short-lived spam-rampage (... spampage?) Nevertheless, I would be willing to pay it.