I don't think the used market value is unique to any one brand and I have a few examples.
In 2015 the MEG 2.5 and 2.7 units ceased production and a new head was made, the MEG 15 head. Though compatible with the other legacy hardware like loop, canister, scrubbers it was something like $5k to replace the head. Users were informed support for 2.5/2.7 devices would go to 2017. Adjusted for inflation that's $6625 today.
I don't know the exact change dates for the Poseidon MK6 to MK7, and when people originally sent them in it seemed to take forever, but I think it was around $3k for an electronics upgrade. Their upgrade process morphed a bit and I didn't follow it but I know the battery pods needed upgrade from say recreational to trimix and it was something like $1200 for just a battery swap because the battery pod included a portion of the electronics. The Poseidon would literally lock you out from diving if it wasn't annually serviced.
The Hollis P2 launched and almost immediately, probably within 2yrs of launch, Shearwater morphed to the DiveCAN system and all the original P2 heads needed an expensive refit if they went back to Hollis. I don't know what the cost was, I didn't send mine, but at the very least it was a new set of Shearwater HUD, Handset, and board plus other service fees so it couldn't have been less than $2500.
I don't think we need to talk about the value of a Hollis Explorer. In 2013 it retailed for $3500, or at least that was the price on the tag on the one I have. I got one literally for free, missing only the cylinder, which I passed on to someone else for free. Effectively these things are now worth nothing except for the pelagic transmitter and the cylinder.
The Sentinel/Redhead changed ownership I think 3 times and honestly I didn't follow it, I have 2 old sentinel to sell and I think they are valued less than $1k for both. It was a huge unit and support outside the UK was minimal at best. These things became essentially boat anchors for divers in the US.
Here in the southeast US we've seen mass adoption of the Choptima and lots of folks dumping AP and rEvo units for a fraction of the price. I sold a trimix upgraded Inspiration Vision, which needs factory service, but otherwise had less than 100hrs on it for $1500 AND it included the original AP Y-BOD electronics/canister/etc and a box with like 40 service kits/tools. A rEvo from a close friend needs factory service and will probably sell for $2500-3500 if he's lucky. Though there is a service center for AP in Florida I don't know if there is a functional service center for rEvo in the US anymore. Why would you buy an old backmount unit which needs an expensive service when you can get a lightly used Choptima (I think the oldest are from 2020-2021) from some Veteran and be all in with service locally for $5-6k?
Currently an electronics upgrade on a Liberty is something like $2200. The company recommends replacement of the lithium batteries every 3 years, a pod which is user replaceable. But older soft goods are always my concern with rebreathers. I once took in, for service, a BC which looked beautiful and well maintained but the corrugated hose pulled right from the bladder. However it was stored the urethane and RF welds had dry rotted and failed. If you buy a used rebreather you expect to have to replace the soft parts like corrugated or especially cooper hoses, counterlung bladders, etc. But none of that is cheap and if it wasn't factory serviced where someone evaluated or replaced those components then that added cost goes into what you are willing to pay used.
Another growing factor is user modifications. Many instructors, probably most in Europe, want you to have an essentially stock rebreather. The manufacturers called for it with RESA and the training agencies have similar requirements. I don't think any instructor will raise hell about a harness change or something minor but they could. If someone buys a used unit and intends to take training from a reputable instructor there is a good chance they will need to pay for things to be returned to stock or updated in some way. If you have say a MEG with backmounted subgravity lungs and a rEvo 3 button MAV block good luck finding an instructor who will train you. This is only going to get worse with ISO training requirements being adopted by essentially all the major training agencies. Instructors have a hard enough time getting insurance, it's not going to get easier, it's in their best interest to follow what mommy training agency asks for.
Lastly you need to factor in trends in the community. Divesoft started with a front mounted counterlung unit and then went backmount counterlungs because of demand. How many people do you see diving front mounted lungs on any platform anymore? It's rare here. If they have something in front of them it's a whole chest mount rebreather. I personally don't see the draw for any of that chestmount clutter, be it lungs or the whole machine. If you have an older unit with a configuration that is in low demand in your region how can you expect to get a high price for it. Someone will want to pay a low price and then make it into a sidemount unit or whatever, essentially only using the head/scrubber. The conversions are something like $4-5k.
It is what it is. Overall I think Divesoft isn't that expensive compared to upgrades from other companies especially not when adjusting for inflation. If you have a local service center, or not, plays a big part in what you'd be willing to pay for a used machine. How much of the device can be, or more importantly should be, user serviced will always be a debate and manufacturers aren't always clearly distinguishing that line. Buying a unit with unknown history is always a gamble, if improperly stored looks can be deceiving and suddenly you have costly replacement parts to buy.
For me a unit which was dove yesterday by the original owner is worth half the cost of current retail ($10,900) as a starting point ($5450). We know it works, but I still want it disassembled and all the seals replaced. Probably the corrugated hoses too cause I don't believe you ever really clean them perfectly from the last user. From Divesoft's website that's almost $1k in service/parts so I deduct that ($4450). If anything isn't stock, or the harness is too small, I'm going to look up those costs and deduct them (too variable but always something). In this case I'd want the 2021 electronics simply for the transmitters so I'm going to deduct that upgrade cost too. Making, in this case, the value somewhere in the range of $2250. However, if it's a front-mounted counterlung unit instead of back-mount, what I'd want, then we need to look at that. Surely I could save on not sending some items for service but the conversion kit from FM to BM is another $1900. For me it's just not worth the investment to buy this unit used since I want too many changes. Not when I could buy a new one with known history and presumably some warranty and pay it off over 12 months interest free.
