Private security officers are cheaper because lots of them are either: 1) Considerably less trained/qualified than police officers, or 2) Police officers working part-time for additional income, whose training, benefits, etc., come through their main job (and who become unavailable when required to perform overtime in that job, for which they are paid more.)

OPD is also very expensive compared to other police departments: http://www.eastbayexpress.com/oakland/throwing-more-money-at...