That would be kind of cool, actually. A $5 one-time fee for every user. It would keep out the riff-raff and spammers, and there would be no need to sell user content or even have ads at all.
Another problem with a $5 one-time fees is that people expect to use your service in perpetuity. That price point probably isn't sustainable over a more than a few year period.
My main point was that a 5$ ONE TIME charge is way too low. Typically ARPU is calculated over a certain period of time (monthly, quarterly, yearly).