Technically, there is no set figure, however, if it can be shown that a component that would have caused the car to fail it's MOT caused or contributed to the incident that wrote the car off, you shouldn't recieve a penny.
We generally deduct 10% if the car has no MOT - at the end of the day, regardless of why there is no MOT on the car, a car with no valid MOT is going to be worth less on the open market than a car with a ticket .
Lot's of cars shouldn't be on the road after having an MOT - the MOT only shows that the car was roadworthy at the time of the test, so you have at least 11 months where you can drive a car around with no lights, bald tyres, etc, so it makes little to no difference as to whether the car should have been on the road or not.