Hi Julia...welcome to the Forum
It is worth looking at other variants of the A3 for replacement parts such as Volkswagen and Skoda as they all pretty much use the same chassis components....may find they are being offered cheaper than Audi parts.
Use websites like http://www.eurocarparts.com/ and http://www.audiownersclub.com/parts-results/A3-S3/Steering-Racks--Boxes/33589
Also, £600 for labour is an horrific cost, shouldn't be more than about 2 hours work tops!
Let us know what you find
Cheers Trevor