Sports diff was £620 to be be precise (I think the S4 and S5 option prices were the same).
Damper control was cheaper at £300 and to complete the set the dynamic steering was £375.
But as noted with any or all of those three options, ADS was mandatory at £220.
@Breezey - in 2014 (it felt like most of that year) I hunted for a used S5 sportback (I'd have accepted an S4 avant instead, but I prefer champagne to sparkling wine
). But after a LOT of test drives, I decided that the car must have both ADS and damper control, plus I wanted a few other options too, so after many months of waiting for the right car to become available I gave up and ordered a new one. I doubt that the situation has changed - if you want sports diff then you may have a bit of hunting and/or waiting to do. I urge you to think carefully about what options to insist on.
If you have time, I suggest you get some test drives of B9s with and without sports diff. As long as the cars are similar otherwise I think it should be enough for you to tell the difference. I can easily tell the difference, as I did during my test drives, but then my driving style allows it. Before I decided on a B8, I test drove a few other makes including a Nissan GT-R. I did my usual cornering tests with that car and could make it lose traction, although in the case of that car it was due to its suspension and tyres and not the diff (it has a LSD). When I tested B8s with and without the sports diff, I could easily make the latter lose traction but the former was much harder. My previous B5 S4 didn't have an LSD which is also why I knew I had to have one on my next car.
So to answer your question from #7, really you need to ask yourself do you need a car with sports diff? I'd say that if you have driven a car without an LSD and disliked how it starts to break away when powering around corners, then you'll benefit from a B8 with sports diff. If not, you probably won't.
Back to your main question. Issues with DSG are such a focus that I tend to forget what other common problems there have been. When I was originally looking I think there were some cases of central locking problems, so check that it all works as expected. I think I also saw a few reports about premature wear of wheel bearings but only a few. Then there is the clutch on the manual. Its not the best and so if its a high mileage car make sure to test it. Finally there is engine oil. You may know about the debacle for the 2.0 TFSI, however I have seen a few reports of high consumption on the 3.0 TFSI too. Again it is rare, and I'm not sure how you can tell when looking at used cars other than to check the oil level - if its low then might be worth worrying about.
But I can't stop without going back to the DSG. I gather that you were bitten by this? For the B8 S4 by far the biggest number of problem reports have been with 2009/10 cars. But we know that Audi made two revisions to the DL501 gearbox. I believe that the first was in 2011. I am not sure when the second was, so I don't think its possible to suggest that there are no problems with the B8.5 'box, and indeed there have been reports on cars made as late as 2014, but a lot fewer than the B8s. The advice has to be to get as new a model as possible. But no matter what the age of the car, the service record is paramount. There is no doubt that the DL501 can suffer if not serviced properly. Audi demand an oil service every 38K miles and there is good reason to argue that this is too long. But if you find a used B8 or B8.5 which missed its s-tronic gearbox service then don't buy unless you have a rock-solid guarantee of warranty.
. The more the number of launches (e.g. 40+) and I’d stay away from that Car. 
