Consider 10 years service life of a car and deduct some percentage yearly.
1st Year- 20% depreciation.
2nd year- 10% depreciation.
3rd Year- 10% depreciation.
4th Year- 5% depreciation.
5th Year- 5% depreciation.
subsequent years- 5% per anum.
Until the price becomes zero.
If there is major accident deduct 20% extra.