I think the 500 is good value, I paid £11500ish for my one nearly 4 years ago and it's worth around £7000 now, thats a loss of 39% in 4 years I guess it has held its value so much is because the price of new ones have gone up.
Don't forget the prices of most cars/things have gone up in that time.
The 500 does not get driven that much at this time of year because believe it or not there are some even more fun cars than the 500, and they also hold there value better 2007 car £85,000ish new now worth £55,000ish so a 33% loss. Again the GT3 basic list price has gone from £80,000 to over £100,000 in 5 years!