BMW have one which should arrive when the Nav 111 appears in a few weeks. Don't know how much it will be, but just going on the handle bar mount price, I`d expect it to be around the £50 mark, but could be cheaper.
Or TT have one,
http://www.touratech.de/shop_uk/enter.html its the "BMW R 1200 GS Adventure GPS adapter" at £23.97. Ignore the actual mount which is not included in the price. Your need a mount as well from here
http://www.touratech.de/shop_uk/enter.html . Works out pretty expensive this way though.
Or a mount from
http://www.motorradconcepts.com/bmwr1200gs.htm#gps01
Or a mount from
http://www.migsel.com/store/index.php?act=viewProd&productId=9 with necessary extra RAM parts.
Pic below is the BM mount.