how much does it cost to replace knock sensor

I had both of mine replaced by a garage in Ft. Walton beach, Fl. in 2010 while on vacation. the cost was $500 of which, approximately $350 was labor. If I had been home, I would have replaced them myself. The Chevy dealership and the "older" salesman at Auto Zone advised me not to wait because I was risking severe detonation and head damage without knock sensors working properly. They are located under the intake manifold and require it's removal. Good luck!!