Car costs around 3 grand a year to keep on the road without problems.
That's petrol (20 mpg is average, £1 a litre, 10k miles), annual service at independent VAG guy (upto £200), insurance (£550), road tax (£190) and MOT (£60). Petrol is by far the biggest cost on the vehicle. But remember this is without parts, extras (say cambelt or tyres) or tuning/mods which also add up.
£400 ish for a cambelt service
£150 for a couple of control arms on last MOT fail
£120 for ABS controller repair/refurb
£600 for tyres
£80 today for another tyre thanks to pothole
£290 for a replacement wheel thanks to same pothole
£170ish for an EGT sensor
£80 on front brake pads (though those are big brakes I have)
Mods I am not gonna go into because it will make me cry LOL
There's nearly another 2k this last 12 months on top of the 3k general running costs that I can recall off top of my head. You could perhaps discount the extra tyre and wheel, but at the end of the day <beep> happens and there is nothing you can do to get round that. Remember you are buying a car which is gonna be around 10 years old, which may or may not have been looked after (and my examples above are on a car which is from another enthusiast owner, a car that really wanted for nothing IMO), that will have worn out parts like suspension, bushes and clutches, due it's second cambelt most likely, ABS controllers about to pack up, EGTs wanting replacement, K03 turbos which could cost you big if they are on their last legs, leaking boost pipes/DVs...
This is not a cheap motor to keep on the road and don't kid yourself that the (relatively) low purchase price of a 98.99.00 plate S4 is getting you cheap motoring because it's not.
But don't let it put you off

The car is a hoot to drive, feels safe, is damn fast, has big tuning potential if you have the money, and puts a smile on my face every single time I plant that accelerator pedal and feel the snails spool up watching yet another <beep> who had no idea that he was about to beaten to submission by a fast blue family estate car

. S4 Nogaro Blue Avant . ECS IV . RS4 handling bits . 18" Ultraleggera . AWE Boost Gauge . MRC Custom Map . 315 BHP . 482 Nm .