I was quoted ?2,750 to retro-fit the towbar (I was looking for a Used Approved Vehicle), which led me to order a new car with the tow bar fitted.
Apparently all the costs on the retro-fit are in the relocation of various parts of the car, replacement rear body panels etc, most of the costs involving the labour element?
I had similar with my A3 which was an ex-demo, but the sales guy screwed up and quoted me the same ?550 as a factory fit option - fair enough they stood on with the deal, but the car had to have a new rear bumper which had to be sprayed, so they definately lost out on that one! BTW, the towbar fitted was a Bosal one in that instance.
I doubt if one of the retro-fit routes will provide the completely hidden version that Audi offer (unless you shell out the ?2k + of course), but if you were happy with a permanent towbar, I am sure someone will have figured one out by now?
Regarding the bike rack, I have the Thule (2 bike version) and think it is one of the best purchases I have ever made - the tilting mechanism which allows you to use the rear hatch without taking the whole thing apart is invaluable!