5$/month for PVP content only...5$/month for questing + dungeons content only...5$/month for raiding content only...
...stuff like that?
I would return for PVP content only for 5$/month only because I REALLY like Paladins and Rogues in BGs, otherwise there's just too many alternatives out there to justify coughing up 15$ every month in return for a few hours of PVP a week.