It depends on the car, how old it is and what price guide you're using. There's no rule of thumb and different guides approach it differently. Unless you've got a degree in maths, it's best to stick to what the guide says rather than work it out yourself. Mileage makes a lot of difference to a car's price in the first five years or so, but becomes less relevant as a car gets older. Condition, service history, MoT and tax make more of a difference on an older car.