Had this servo failed message along with auto stop/start appear on my A4 Avant. Purchased the replacement part from Audi as I needed it in a hurry and it cost me £108 inc VAT for the part. Fitted in less than two minutes following a YouTube video.
The technician at Audi who served me could not believe the price and also commented that they had lots of issues with this sensor. I would imagine this is actually a £10 to £15 part to produce.