Glass's guide is always seems lower than what you'd expect to see on autotrader.
Also, they don't seem to have the 313s listed, it appears they do on that search, but if you try it on the actual Glass's guide it shows them as 296bhp... could just be an error though.
It thinks my '07 313 Roadster with 16000 miles is worth £12,600... the only equivelant I can find on autotrader is for £16k, or £15k for 1 with 28k miles and no sat nav.