As an average with varied driving (town work, motorway etc) I was getting 18. In fact I did 3000miles around Europe which was a mix of motorway (at 150mph), back roads, lots of Alpine hills and 4 days at the Nurburgring and I got just over 18.
Now the wife isn't driving it across town each day for work, it's gone up a bit to 19.
I worked out how much my fuel bil was on my chipped TT and the mpg and then worked out how much difference it would be in the S4. Given my milage it was something like £50 extra so that was acceptable to me.