You have to work either expected life by age, or by miles/km.
Purchase price new = $30k; expected life = 300,000 km or 15 years, whichever comes first (but assuming neither is ridiculously off - so a 20 year old car that's only done 20k km will not fit this thinking).
If you live in a salty area, I would put that 15 years as a hard cap. If you live in sunny California, and keep the car in the shade (garage), it could still be just fine in 25 years.
So a 3 year old car that has 60k km (or 40k miles, with cap of 200k miles) should 'straight line' be 80% the price of a new one. With some wiggle room; a used car at 3 years will have little or no warranty left, and 'who knows how the previous owner took care of it'. So if it's close to 80% I'd go new. 70% or less, used.
Subies don't depreciate fast, though. People love them. Same as Toyotas and Hondas.