There is two different thermostat types, hence the difference in price etc.
Version 1: It is possible to just replace the thermostat valve.
Version 2: The thermostat & housing are a sealed component and hence to replace the thermostat you also have to replace the housing too. It is not possible to open it up due to it being plastic welded together.
For what it's worth mine (2006 A3 20T quattro BWA eng. code) had version 2.
My advise is, to echo the above comments, go to the main dealer and just give them your VIN/REG number and they will sort it out for you.