Bought mine for £6150 (52, 33K) two weeks ago off autotrader if thats any help. Saw yours on here (and pistonheads too?) when I was looking. Seemed a fair price and condition to me, the distance away was the only reason I did'nt consider buying yours.
I think the thing with the cup is that it is fairly specialised, in that only enthusiasts really understand what it is. Joe Public generally does'nt, and so just sees its lack of toys / driver aids / leather trim as a reason to choose a standard 172 over a cup. Although of course the cup does look the nuts over a standard 172!