The RS6 is overall an expensive to run performance car; obviously an excelent car but expensive all the same. Common costs include:-
4 x tyres at around £1000 per set... every 10K miles approx
4 x breaks (the price quoted above sounds cheap).
petrol @ around 15 - 20 MPG
Servicing (not sure how much, haven't paid for one on mine yet)
Belt change every 40K miles I think £500 - £800.
These are just the costs I can think of at this time, I'm sure other people can add further running costs! Not to take account of the deprication and loss of interest on the capital....