To answer why it's so expensive.. it's just an easy way to make big money.. most people all just pay it thinking yes Audi knows best. Exploitation is what I call it.
No idea if it's the same part number.. you can just call the parts department and buy the sensor for your car. Nothing wrong with that. I'm guessing it will be in the same place. Worst case scenario is you have to go to a garage and ask.. surely won't be as much as Audi. Heck be nice to the guy in the parts dept and he'll be able to tell you where it goes from their parts diagrams.