Welcome to the forum Dave, you'll find the members on here are a friendly and helpful bunch :)
Yep it's a common issue on various vag cars from this era including the a4, a6, golf, Passat amongst others. They rust because vag put foam inserts between the wing and wheel arch liner which obviously fill with water and thus, cause the rust.
I had my wings replaced 18months ago and it cost me around £1000 so I'd say this is fair (I can look through the invoices to see exactly how much if you're interested) that price you'll be looking at pattern wings (non Audi) and the cost will include the wings, primer, paint, lacquer, blending in and labour.
If the cars a good one then I'd say have them changed :)
Hope I've helped :)
Cheers
Steve