The wiring harness for the parking sensor was being added in late July '13. So if you build date was after 1 Aug, you can count on the harness being there.
I have two friends here that in 2014, had the parking sensor added to their Model S. The cost was $4,000 + tax. Ask again and get it in writing. I suspect the price will be some where close to the figure we saw before by my Tesla owner friends.
GA24, are you also- separately referring to the front mounted camera that connects to the center console? Tesla will not install it. But therre's an effort underway with people here on TMC to make the installation a lot more user friendly.