This is a great topic Richard, thanks for starting it!
I have a couple of stories from the UK that I can share. First of all, we're going back a little over two years when Justin was looking to trade in his 2013 S for either a 2015 or 2016 SR. I was interested in buying Justin's old bike so we had a chat about prices. IIRC, Justin was willing to sell his bike for around £5,500 privately. I think that price may have included the soft panniers, his Elcon and the cables he has made up. His bike was only about two years old with around 20,000 miles on the clock and in good condition. Or was it £4,500 we settled on? I'm not sure. Two years is a long time. In the end, I got an opportunity to buy one of the 2014 DSP bikes for £6,500 and that was that. With the benefit of hindsight, I'm glad I did. The two year warranty I got on buying an unregistered bike has saved me a lot of money due to reliability issues.
I'm pretty sure Justin only got around £3,000 for his trade-in. That sounds terrible considering the bike would have retailed at around £12,000 new in 2013. However, bear in mind that he may have got a good deal on the 2015SR he bought to sweeten the deal. I don't know more than that and it would be remiss of me to divulge all the details if I did. After all, that was Justin's business.
I do seem to recall that Justin's 2013 didn't go on sale. I'm pretty sure that it was absorbed by Zero. There may have been a defect along the way too. I'm sure Justin will clarify if he sees this post and has the time.
Also, keep in mind that depreciation on a new bike is partly due to tax. In the UK, we have 20% VAT (sales tax). So a bike retailing for £12,000 has had £2,000 of tax applied to it. As soon as you've handed your money over and ridden it out of the showroom you aren't going to see that £2,000 again. The tax man doesn't do refunds. VAT is also part of the reason a second hand bike costs more from a dealership. There's something called the Margin Scheme in the UK that means a dealer only has to charge VAT on the margin they sell a second hand bike for. However, it could still add a few hundred pounds to the price.
The other story I have is my own. A few months ago I asked my local dealer what they'd want for the 2016 DSR demo bike they've had for about a year or so. They answered my question with a question and asked me what I wanted for my 2014DS. I scratched my head for a moment and said £5,000. The salesman said he'd have to prepare some figures and he'd give me a call. The call never came...
My gut feeling is that I'd be lucky to get £5,000 for it privately. I would probably only get £3,000 for a trade-in. Again, that's shocking for a bike that retailed for £12-13,000 in 2014.
I was fortunate enough to get an extra year's warranty out of Zero after the fiasco with the bearings in my rear wheel. The bike also still has 3 of its five year warranty left on the battery. That helps the resale value of my particular bike at least. If anyone wants to make me an offer, let me know!