For the average retail prices, go to Kelly Blue Book or Edmunds and enter the necessary data. If you search nationwide on Cars.com or Autotrader you will usually find a TTRS with about 10K miles for the high 40s being sold somewhere by a new car dealership that is not Audi. Audi dealers are asking premium prices for used TTRS in an attempt to artificially increase market value. Of course, the non-Audi sale does not have the CPO warranty but still has the remainder of a factory warranty. Then when the factory warranty expires, you can buy an extended warranty better than the CPO for less money than the premium price Audi is asking.
Today, Cars.com shows a used car dealer in Thousand Oaks CA is ASKING $46K for a similar TTRS with 19K on the ticker.
Unfortunately, the TTRS you are interested in has only been on their lot for a few days, so they are not likely to be very negotiable. I bought my latest TT used from from Beverly Hills Audi in December of 2012 and they were a pleasure to deal with but it took 6 weeks of on and off negotiations before I got the price I wanted. They wanted $40K, I paid $34K (I started at $33K).
The TT and TTRS are great cars but have not sold well, so if you are patient you can find a great deal somewhere because they usually stay on the lot a long time. Call your local Audi service manager to get the repair history before you buy any Audi. Most will be glad to help you.