Post article says external bus cameras are $3,500

 ...Montgomery has tested the external cameras since 2009, monitoring vehicular movement to the left of the bus when its ignition is on. Warning letters were issued to the owners of violating vehicles, he said. It costs about $3,500 per bus to install equipment used in the test...

http://www.washingtonpost.com/local/md-politics/montgomery-proposes-using-cameras-on-school-buses-to-catch-safety-violations/2011/11/21/gIQA5W0QAO_story_1.html