The MSRP of a performance ATS without extra options would be around 42-44k. But don't let those numbers fool you. There are heavy incentives, and very few have paid MSRP. Please look at this link:
https://www.truecar.com/prices-new/...108854,4000110454&trimId=259589&zipcode=08201
I entered your zip code to true car value, and it gives the average MSRP, invoice, average paid, and the estimate that the TRUECAR gives as the price that it might find for you. As you see, in average people are paying 7k below MSRP. To be more precise, the averaged paid is $37,443 for a brand new 2.0 liter turbo ATS performance. Forget MSRP!
When you go to autotrader, they list the ASKING price. $34k for a used '13 ATS is a joke! If you think or if your gut tells that this two year old car is worth $30k-$31k, that's fine. As rustybear mentioned, pay whatever YOU are willing to pay. But IMHO, $30k-$31k is too much. The retail should be well below 30k. But 27k seems to be a good number if you want to buy from a dealer and you are sure about the condition of the car. And by the way, why this ATS ended up on an auction?!