As I think I've said before, values are holding up on good quality products which are still
reasonably current.
That's why the most recent TB20s (2001/2002) still fetch good prices. And I
am taking into account the fact that most sellers don't like to admit how much money they lost on something...
Recent Mooneys will also fetch good prices.
It is the 1980s production stuff which has bombed. For example you can buy a 1985 TB20 for about £40k. It's going to be a dog with loads of things that need doing but so long as it legally flies it's still a TB20... That same plane would have been £60-80k 2 years ago.
Cirrus values have dropped more because they have been continually changing their specs, so their "most recent" offering is actually really recent, and something say 5 years old is going to appear well out of date. I should think they Avidyne models have well bombed since the G1000 ones came out.
Socata and Mooney will always be around making the parts because of the fleet sizes (~2k for the Socata TB; no idea of the Rallye fleet size) and since the TC owner can generate the paperwork from fresh air, it's a massively lucrative business. A 3rd party parts maker would need to get an STC for most parts which would be silly. Look at Piper - they are selling very few planes these days, but have a huge worldwide parts operation. Only an idiot will run a business like that into the ground and leave it there, so the TC is lost.
the TB20 is a great aircraft if a little slow in this company and dated
I think you will find the TB20 does the same speed, at a given flow rate, as an SR22 or a Cessna 400. Mine does 165kt IAS flat out but who wants to be burning 20GPH? The SR22 is ~10kt faster than the TB20 at say 75% power but you pay for it in fuel. Dated, yes, the latest avionics are the late-1990s stuff, but it's the least dated airframe of all the non-composite airframes in IFR GA.
A glass cockpit means you are over the barrel on any significant avionics work; OK if you have the facilities handy (and don't fly too far away) but a right pain otherwise. The one thing I have noticed recently is that the installation and maintenance manuals for these are very tightly held. Garmin have had their legal team hitting all websites which carried any of their maint/install manuals; too late as the manuals for most of the pre-glass avionics are all over the place (I have a huge collection myself but obviously not on an open website) but they have evidently been successful in keeping the G600/G1000 manuals out of circulation and restricted to the authorised dealers only, so even simple stuff needs to go to the pukka dealer. I would hate a deal like that. Honeywell are the same in theory but in practice they have not really bothered to keep the stuff tightly held, so most avionics shops (or freelancers) can work on the 1990s stuff and older. Autopilots are the exception as they need some weird adaptors / bus extenders (which the dealers have to buy for £xxxxx in order to qualify for the dealership) but otherwise setting them up is trivial enough.
With a 1990s outfit if an instrument goes you pop in an exchange unit and off you go. If however you are tied to a specially authorised dealer and (generally in light GA) you have to go to him, it's a whole day wasted each way.
The reality is that with most planes one gets little "issues" and the owner will eventually get fed up with what he sees as avoidable hassle, so I think glass cockpit planes (which are frankly not flyable with major cockpit defects except under VFR) will depreciate faster. I know of a number of people who would like to dump the stuff they recently bought, and some of the "stuff" I am talking about is well above the piston level. Technology is good and glass cockpits are the best way of presenting the information but not with the way the maintenance scene has been tied up. It's not so bad on jets because the maintenance people travel to the customer but in piston GA you have to make the trip every time.