I got my RNSE of Ebay just be cautious about who you buy from as most are from germany even if they say UK on Ebay.
I think PJMSpeedy can get them for about £450 so you may wanna drop him a PM. I didnt need VAGcom to fit mine as its pretty much plug and play if your car is already double din.
Also when you buy the RNSE check the model number as this will tell you how old the unit is and if its an A4 model eg :
8E0 035 192 C Audi A4 Europe, MY 2004, 8E B6
8E0 035 192 D Audi A4 Europe, MY 2005, 8E B7
8E0 035 192 E Audi A4 North America, MY 2005.5, 8E B6
8E0 035 192 F Audi A4 North America, MY 2006, 8E B7
8E0 035 192 G Audi A4
BNS 5.0
8E0 035 192 H Audi A4 Europe, MY 2006, 8E B7, with SDS
8E0 035 192 J Audi A4 North America, MY 2006, 8E B7
8E0 035 192 K Audi A4 Europe, MY 2007, 8E B7, sw 0500
8E0 035 192 M Audi A4
BNS 5.0
8E0 035 192 N Audi A4 Europe, MY 2007, 8E B7, with SDS
8E0 035 192 P Audi A4 North America, MY 2007, 8E B7
8E0 035 192 Q Audi A4 Europe, MY 2008, 8E B7
8E0 035 192 R Audi A4 Europe, MY 2008, 8E B7, with SDS, sw 0650
8E0 035 192 S Audi A4
BNS 5.0
8E0 035 192 T Audi A4 Europe, MY 2008, 8E B7, sw 0650
8E0 035 193 Audi A4 Europe, MY 2008.5, 8E B7, with SDS
Most of the RNSE`s on Ebay come bundled with the other bits you need to make it work but make sure it has : the GPS antenna, Stereo removal keys x 4, Wiring loom adapter, an Audi map DVD (as new as poss and preferably an original disc).
Lastly if the screen looks scratched (the anti gare coating wears off) this can be sorted so if you can get a cheaper one that this has happened to you can buy some Meguair PlastX (
http://www.cleanyourcar.co.uk/engine-amp-exterior/meguiars-plastx/prod_283.html) i used it on mine and it looks as good as new now.
Hope that helps