With video cards, in a normal market to which we are slowing ebbing towards, the used resale value curve tapers off over time, and there's a base functional utility to consider that stabilizes value.As a proxy for how cheap older GPUs should have been, I saw Nvidia K80s listed for like £180 recently, AFAIK these aren't desirable for miners and most gamers won't have flocked to them as alternatives so IMO they probs haven't been subject to the price inflation of other cards, they were like $5000 USD back in the day brand new and were still pricy in like 16, 17 etc. IIRC.
Arguably shouldn't stuff like the 1080ti, Titan Xp, 2080ti etc.. have been at more like sub £200 or less by now too? (If it weren't for miners and the knock-on effect of supply issues for gamers)
As for the 1080 ti, it still generally outperforms a number of modern cards that are ~£300 so it's value has really only been degraded by its "oldness".