£550 sounds a hell of a lot.. but sounds about right for the OEM sensors, fitted by a dealership.
You definitely want to make sure they're talking about the OEM sensors though as Audi actually fit retrofit Cobra sensors too, which look like this:
Obviously totally different to OEM!
You can get flush parking sensors for around £199 fitted. Aftermarket but look completely OEM and for more than half the price Audi quoted you.
They look like this: