In what way? Repairs if something goes wrong maybe, but nowhere near as much as a BMW, after all, it's the same 2.0l lump that VAG use in the golf, octavia and leon amongst other cars. £300 for a cambelt change etc. I'm getting very close to 50mpg in mine. It isn't fast by any means, but i'm happy with mine, had it almost 6 months now and it hasn't missed a beat, again, no added costs so far but am due a couple of front tyres which will cost me just over £300. Tax isn't too bad either. Services every 20k miles as well.
To OP, as above, i've had my 2.0tdi Sline (albeit not avant) almost 6 months, it's been perfect so far, see above for my economy etc. The one thing i noticed when i first drove it, the steering is very light! It isn't a drivers car in that respect, but i enjoy driving mine still. You get more car for your money than you would from an A4, look out for MMI 2G/3G high (colour nav etc), i think it's essential in this car and you get it at no real premium in a used car. I love the S-line seats too. And look out for DAB, Bluetooth and Audi AMI (music interface (it'll be in the glove box if you get it)). Any more questions just ask