If you are worried about running costs forget about it.
I think the fact that the prices have gone down had meant that people have bought them that can't afford to run them.
Then they don't get looked after well and it makes it harder to find good examples.
Put it this way. If you can't afford to spend on average £800-£1000 per month on a car (not including buying it) then forget about the RS6.
If you don't believe me then do the math. Fuel + Insurance + Tyres + Discs + Warranty + upkeep + servicing + extras due to wear and tear then divide by 12. Mine came to £1000 per month for the first year and £800 for second. My very first service bill was £1400 and that was not Audi (would have been £2000) - had to get timing belt kit plus extra plus pads all round plus normal service.
I love my RS6 but people need their eyes open when they buy one and don't want to see any neglected
Steve