The general answer is that depreciation, very low in the early part of this decade, has accelerated and is now rapid.
A lot of people are saying the plane market is going like the car market.
A lot of the "blame" in the £259k market is of course Cirrus. You can't sell thousands of new planes, especially marketed specially to people who are not flying anoraks, without a fair proportion of them ending back on the market.
In Europe, Diamond have done their bit for prices too, but got a bit of a bad name with the engine issues, so depreciation on them is high too.
The glass cockpit situation is another factor - you can't give away a new plane without glass, and this depresses used values (wrongly IMHO since a glass cockpit does not give any extra mission capability over decent current avionics).
It's a good time to buy a used IFR tourer - for the long term. I now see some 1980s TB20s advertised for £40k - even though these are obviously old dogs, this is amazing since you could buy one and thoroughly refurbish it and still have a very capable machine.
A £200k metal plane bought in 2002 is now worth about half that.