We have used the guardian angel feature for members who may have caused a problem and we need to track their activity to either be sure they are not doing the same thing again. Also it is a good way to track members whose background check shows an area that you would not want to subject them to temptation.
Cost is TBD
Cost is TBD depending on any additional levels of notifications, permissions, etc that are requested beyond those already built into the Drupal system.