Look in Parkers used car guide. Depreciation for 'trade in' purposes 50% in the first six months.
I drive a couple of Mk2 Nissan Micras for everyday use. As depreciated to zero by now. The other thing has a 5.5ltr V8 and weighs a couple of tons. Still returns over 25mpg. That's depreciated as much as it can as well, within reason.