Depreciation is affected more by mileage than age, I assumed a trade in value after 4 years of £4k. So if the screen price is £24k, and the final value is £4k, that is a loss of £20000, which over 4 years is £5k per year
I also assumed 40k miles per year, so after 4 years, that's 160k. If the annual mileage is higher, then the worse the depreciation gets.
At that mileage insurance, tax, servicing and tyres come to a whopping 7.5ppm, easily the smallest cost of the car.
Finance 18ppm
Fuel 15ppm
Depreciation 12.5ppm
Ins/tax/maintenance 7.5ppm
Regarding your 600CL example, I bet he didn't pay them to fit them though
Anyway, I said I assumed alot, put in the actual dealer figures, based on the expected annual mileage and see what comes out. Until you actually run a particular car, you won't know what the brake or tyre wear rates will be, so overestimating is actually good practice...