there are many different around....like some kit has 1 sensor while other has 4 sensors....some have just beeping sound while other has an led screen show how many feet you left.......also so of them require drilling hole on your bumper cover and other just hide inside the bumper cover......
example: i of my friend have a kit (with 3 sensor, beeping warning and lcd screen show how many distance left, and drill hole on the bumper type) installed in his 03 accord sedan from my other friend's garage in thronhill for roughly $250
another example: just get a quote from hong kong for a complete set of Toyota OEM parking sensor kit (dealer accessories in Japan, with 6 sensors, 2 on the front bumper and 4 on the back, with beeping sound warning and screen for showing which sensor sense obstacle) for my 2001 prius for almost CAD$1000! (OUCH!)