Possibly what the OP is referring to is the trade in price.
To my understanding government policy put a heavy onus on the trade to sell new EVs, not used EVs. So the trade in is unwanted by the dealership as it detracts from chances of selling a new car (and hitting gov targets and avoiding fines). So they dealership offers the owner a derisable trade in figure.
But a dealer being a dealer they'll take the car wack on a markup and stick it somewhere.
Stick a 3yr old Taycan on webuyanycar and it'll give you a ballpark figure.
[click click] just did that and on a low mileage 4yr old Taycan they offer £40k (I know they'll offer less on the day but as a ballpark normal trade in). On a car that was probably specced at about £95k new that's a 58% loss in 4yr. Not great for a Porsche but not unusual or excessive for most car brands.