Where did you get a 20-30% average depreciation on cars? Depreciation is not linear on vehicles. Most of the depreciation occurs in the first few years and then tapers off. Is this calculated into your algorithm?
Kudos for giving us the option. May be helpful flag inputs that are highly deterministic of the output and suggest look-ups.
So if a car starts out at $20,000 it will be worth $16,000 the next year, $12,800 after the second year, and $10,240 after the third. Notice that around the third year, the decrease in value slows considerably.
That sounds like a fair way to do it.