In a recent note, Piper Jaffray analyst Gene Munster said he estimates AT&T is paying Apple an average of $18/month for each iPhone subscriber, or $432 over the course of a two-year contract. Munster revised his previous estimate of $6.50/month based on iPhone sales and revenue figures disclosed in Apple’s fourth quarter earnings report. Munster explained that he was surprised by the higher estimate, and said that it showed the importance of Apple’s efforts to keep the iPhone out of unlockers’ hands. Apple recently instituted a new iPhone purchase policy, limiting sales to two per person and barring cash sales in an effort to reduce the threat of unlockers. Based on the estimate, Muenster boosted his Apple 2009 revenue estimate to $42.8 billion, and raised his price target on Apple from $220 to $250.