I think its just because Audi are running out of them. The price has been climbing steadily over the last 3 years or so.
When i first looked, they were about 50quid new from the dealers, they're now well over 100, and presumably they'll continue climbing.
Once the car reaches 10 years old the manufacturer is no longer obliged to sell parts for it, so it'll be a case of when stocks run out they run out.