The dealer is entitled to claim a reduction in the value of the vehicle based on the time elapsed and also the mileage covered, but there is no hard and fast rule around this. Although you have been through a great deal of disruption because of the problems, this calculation does not take this into account, and is purely based on the loss of value in the car. You can try and negotiate this figure with the dealer, but as with any negotiation it is important to be realistic in order to get a resolution.